Quantum Mechanical Fragmentation Research Articles

Angular momentum as a probe for the reaction mechanism: The [formula omitted] decay at three excitation energies

The angular momentum effects on the reaction mechanism in the decay of a hot system, formed in 48Ti + 40Ca → Mo⁎88 reaction at three excitation energies, have been studied using the dynamical cluster-decay model based on the quantum mechanical fragmentation theory. Angular momentum windows have been defined for the compound and non-compound nucleus processes (like deep inelastic collisions etc.) with respect to the angular momentum variation of the fission barriers of the incoming channel. First, the angular momentum distributions of the summed up preformation, penetration probabilities and the cross-sections for light particles, intermediate mass fragments and fission fragments have been calculated by considering the angular momentum up to the critical value. Then, the mass and charge distributions (with isotopic compositions) of the cross-sections have been calculated in accordance with the defined angular momentum windows. The cross-sections for fragments coming both from the compound nucleus decay and from other processes have been calculated and the total reaction cross-section is compared with the one presented in the work of Valdré and co-workers. The ratios of the observed fusion-evaporation and total fusion cross-sections to the total reaction cross-sections of (i) our calculation and (ii) presented in the work of Valdré and co-workers, are compared and found comparable. The change of the reaction mechanism has been analyzed as a function of the angular momentum variation of the cross-sections of the fission fragments. Finally, the effect of the incident energy on the compound nucleus formation and survival probabilities has been studied.

Systematic decay analysis of 202Po∗ compound nucleus using Dynamical Cluster-decay Model

The decay analysis of [Formula: see text]Po[Formula: see text] compound nucleus (CN), formed via [Formula: see text]Ca+[Formula: see text]Gd reaction, with inclusion of additional degrees-of-freedom, i.e., the higher multipole deformations, the octupole ([Formula: see text]) and hexadecupole ([Formula: see text]), the corresponding “compact” orientations ([Formula: see text]), and noncoplanarity degree-of-freedom ([Formula: see text]0), is investigated within the collective clusterization approach. The Quantum Mechanical Fragmentation Theory (QMFT)-based Dynamical Cluster-decay Model (DCM), wherein the point of penetration [Formula: see text], fixed via the in-built neck-length parameter [Formula: see text] in [Formula: see text] (equivalently, the “barrier lowering” [Formula: see text]), is used to best fit the channel cross-section ([Formula: see text]) and predict the quasi-fission (qf)-like nCN cross-section [Formula: see text], if any, and the fusion–fission ([Formula: see text]) cross-sections. We also look for other target-projectile (t-p) combinations for the synthesis of CN [Formula: see text]Po[Formula: see text].

Analysis of 113In(α,n)[formula omitted]Sb reaction at astrophysically interesting center-of-mass energies and synthesis of 117Sb⁎ within the Dynamical Cluster-decay Model

The Quantum Mechanical Fragmentation Theory (QMFT)-based Dynamical Cluster-decay Model (DCM) analysis of compound nucleus (CN) 117Sb⁎, formed in 4He+113In reaction is done at some astrophysically important energies, which is found to decay via 1n-emission to ground state (g.s.) and metastable state (m.s.) of 116Sb. It is for the first time DCM has been applied to analyze decays to metastable states in a “cold fusion” reaction. The DCM cross sections σ1n are calculated at various astrophysically relevant center-of-mass energies (Ec.m.=9.66–13.64 MeV) corresponding to available experimental data for α-induced reaction with the odd-Z, proton-rich (p-) nuclide 113In, measured with the activation method where, in addition to the radioactive alpha-capture (α,γ), some other reaction channels such as (α,n) and (α,p) of the same isotope can be measured at the same time. The only parameter of DCM is the neck-length parameter ΔR, that is optimized to best fit the experimental data for g.s. to g.s. and g.s. to m.s. decay of 117Sb⁎ CN and predict the cross sections for evaporation-residue (ER), intermediate mass fragments (IMFs) and fusion-fission (ff) fragments. For a best fit to the experimentally observed σ1n (ground and metastable states), we find that, ΔR-values vary smoothly with CN excitation energy ECN⁎ and thus is useful for making predictions at energies where experimental data is unavailable. Possible synthesis of this medically important radionuclide 117Sb⁎ is also studied via all the possible “cold” target-projectile (t–p) combinations, identifying 6Li+111Cd as the best possibility.

Fine structure effect among heavy-ion induced fission fragments at near and above barrier energies

The fission decay analysis of 201Bi*, 206Po*, 212Rn*, 216Ra*, 227Pa* and 228U* nuclei produced in 19F-induced reactions at near and above barrier energies is carried out within the framework of dynamical cluster-decay model (DCM) based on quantum mechanical fragmentation theory (QMFT). The interplay between two modes of fission, i.e. symmetric (SF) and asymmetric (aSF), in the fission fragment mass distributions is investigated. For the present set of calculations the SF to aSF peak ratio is less than unity ($ \frac{P_{SF}}{P_{aSF}} < 1$), which in turn suggests the dominance of asymmetric fission for above mentioned compound nuclei. The role of shell corrections and deformations of fragments in the fission dynamics is duly addressed. The calculated fission cross-sections show nice agreement with the experimental data, except for few energies above the Coulomb barrier. This disagreement is associated with the possible presence of non-compound nucleus (nCN) fission as higher energies are more prone to the nCN process. The study of fission fragment mass distributions is further extended to the isotopic analysis of above mentioned pre-actinide and actinide nuclei at common centre-of-mass energy $ E_{c.m.} \approx 80$ MeV near the Coulomb barrier. It is observed that the lighter isotopes exhibit symmetric fission distribution, whereas heavier ones prefer to decay via asymmetric path. A transition between SF and aSF occurs around fissioning nuclei with $ N/Z \approx 1.4$, showing the triple humped mass distribution suggesting comparable contribution of symmetric and asymmetric fission. Finally, the most energetic asymmetric light ($ A_L$) and heavy ($ A_H$) fission fragments are identified for a long isotopic chain of pre-actinide and actinide nuclei. Interestingly, the heavier asymmetric fission fragments are located near a proton number around $ Z=50$ for the pre-actinide region and $ Z=54$ for actinide nuclei.

Investigating the fusion enhancement for neutron-rich mid-mass nuclei using the dynamical cluster-decay model

The mechanism involved in compound nucleus formation through heavy ion induced fusion reactions at near- and sub-barrier energies has been stimulating the interest of both experimentalists and theoreticians. The study of fusion reactions is important not only for the production of superheavy elements but also to unfold the mysteries involved in the evolution of neutron stars and nucleosynthesis of elements in astrophysical scenarios. The detection of gravitational waves originating from the merging of two black holes, and the possibility of observing similar events originating from black-hole--neutron-star mergers, highlighted the necessity of understanding the behavior of neutron-rich nuclear matter. One of the potential methods to understand the character of neutron-rich matter is through the study of fusion of an isotopic chain of reactions. Recently, the first experimental evidence of fusion enhancement at near-barrier energies for neutron-rich nuclei was reported for the light and mid-mass regime. To understand the dynamics involved in such reactions, the investigation of K-induced reactions has been performed using the quantum mechanical fragmentation based dynamical cluster-decay model (DCM). The experimental fusion cross sections of $^{39,47}\mathrm{K}+^{28}\mathrm{Si}$ are reproduced using DCM and, further, the fusion cross sections are predicted for other isotopes of K beam on $^{28}\mathrm{Si}$ to investigate the fusion enhancement as a function of neutron number. It has been observed that fusion enhancement is significant at near-barrier energies for neutron-rich nuclei. The fusion enhancement observed is larger for odd neutron number nuclei as compared to adjacent neutron nuclei. This indicates the influence of unpaired neutrons on fusion cross sections. Also, the fusion cross sections predicted in the present work will act as input for planning more experimental measurements.

Study of α-induced reactions forming A = 60 compound systems within dynamical cluster-decay model

The quantum mechanical fragmentation theory (QMFT)-based dynamical cluster-decay model (DCM) has been applied to predict the fusion cross section (σfus) for the compound systems (CS) 60Zn⁎, 60Ni⁎ and 60Fe⁎ formed, respectively, via 4He+56Ni, 4He+56Fe and 4He+56Cr, reactions, by fixing the only parameter (neck length ΔRemp) of the model, empirically with the available experimental data on σfus of 44,48Ti⁎ and 68Ge⁎ formed through 4He induced reactions at different incident energies, i.e., Elab∼ 10, 13, 17 MeV. We have investigated the effect of neutron to proton (N/Z) ratio in the decay of CS under study, i.e., 60Zn⁎, 60Ni⁎ and 60Fe⁎. The contributions of light-particles cross section (σLPs), intermediate mass fragments cross section (σIMFs) and symmetric mass fragments cross section (σSMFs) are taken together to calculate σfus (=σLPs+σIMFs+σSMFs). The small contribution of SMFs is seen in σfus, the contributions of LPs and IMFs yields being much more prominent, for the decay of all CS under study at different incident energies. We see that the preformation probability (P0) and penetrability (P) for SMFs decrease with increase in the value of N/Z ratio, and hence the symmetric breakup drops-out-of-favor for higher N/Z values. In other words, the symmetric mass decay is favored in the case of 60Zn⁎ having N=Z, the LPs, IMFs and SMFs yields increasing with increase in incident energy.

Different Positron Emission Tomography Tau Tracers Bind to Multiple Binding Sites on the Tau Fibril: Insight from Computational Modeling.

Using the recently reported cryo-EM structure for the tau fibril [ Fitzpatrick et al. (2017) Nature 547, 185-190 ], which is a potential target concerning Alzheimer's disease, we present the first molecular modeling studies on its interaction with various positron emission tomography (PET) tracers. Experimentally, based on the binding assay studies, at least three different high-affinity binding sites have been reported for tracers in the tau fibril. Herein, through integrated modeling using molecular docking, molecular dynamics, and binding free energy calculations, we provide insight into the binding patterns of various tracers to the tau fibril. We suggest that there are four different high-affinity binding sites available for many of the studied tracers showing varying binding affinity to different binding sites. Thus, PBB3 binds most strongly to site 4, and interestingly, this site is not a preferable site for any other tracers. For THK5351, our data show that it strongly binds to sites 3 and 1, the former one being more preferable. We also find that MK6240 and T807 bind to site 1 specifically. The modeling data also give some insight into whether a tracer bound to a specific site can be replaced by others or not. For example, the displacement of T807 by PBB3 as reported experimentally can also be explained and attributed to the larger binding affinity of the latter compound in all binding sites. The binding free energy results explain very well the small binding affinity of THK523 compared to all the aryl quinoline moieties containing THK tracers. The ability of certain tau tracers, like FDDNP and THK523, to bind to amyloid fibrils has also been investigated. Furthermore, such off-target interaction of tau tracers with amyloid beta fibrils has been validated using a quantum mechanical fragmentation approach.

The Three-Body Structure of 2n and 2p Halo Nuclei

A three-cluster model developed for ternary fission studies has been applied for the first time to study the three-body structure of 2n and 2p halo nuclei. For the experimentally known 2n, 2p halo nuclei, all possible ternary fragmentation potential energy surface (PES) is calculated. The two-body breakup reported earlier, clearly indicated a strong minimum in the PES corresponding to 1n/1p and/or 2n/2p cluster plus core configuration. However, the present calculations of PES reveal that, the three- body breakup does not result always with 2n and/or 2p as a cluster. A 1n and/or 1p cluster along with the core is initially formed, and then the core loses one nucleon to make either a 2n plus core or 2p plus core structure. The results are substantiated with the calculations of preformation probability calculated within quantum mechanical fragmentation theory.

Clustering aspects in 20Ne Alpha-conjugate Nuclear System

The clustering aspects in alpha-conjugate nuclear system 20Ne has been investigated comparatively within microscopic and macroscopic approaches of relativistic mean field theory (RMFT) and quantum mechanical fragmentation theory (QMFT), respectively. For the ground state of 20Ne, the matter density distribution calculated within RMFT, depict the trigonal bipyramidal structure of 5α’s and within QMFT, the equivalent α+16O cluster configuration is highly preformed. For excited state corresponding to experimental available energy, the QMFT results show that in addition to α+16O clusters, other xα-type clusters (x is an integer) are also preformed but in addition np-xα type (n, p are neutron and proton, respectively) 10B clusters are having relatively more preformation probability, due to the decreased pairing strength in liquid drop energies at higher temperature. These results are in line with RMFT calculations for intrinsic excited state which show two equal sized fragments, probably 10B clusters.

Formation and decay of the compound nucleus Th*220 within the dynamical cluster-decay model

Background: The radioactive $^{220}\mathrm{Th}^{*}$ compound nucleus (CN) is of interest since the evaporation residue (ER) cross sections are available for various entrance channels $^{16}\mathrm{O}+^{204}\mathrm{Pb}$, $^{40}\mathrm{Ar}+^{180}\mathrm{Hf}$, $^{48}\mathrm{Ca}+^{172}\mathrm{Yb}$, and $^{82}\mathrm{Se}+^{138}\mathrm{Ba}$ at near barrier energies. Within the dynamical cluster-decay model (DCM), the radioactive CNs $^{215}\mathrm{Fr}^{*}$, $^{242}\mathrm{Pu}^{*}$, $^{246}\mathrm{Bk}^{*}$, and $^{254}\mathrm{Fm}^{*}$ are studied where the main decay mode is fission, with very small predicted ER cross section. $^{220}\mathrm{Th}^{*}$ provides a first case with experimentally observed ER cross section instead of fission.Purpose: To look for the optimum ``hot-compact'' target-projectile (t-p) combinations for the synthesis of ``cold'' $^{220}\mathrm{Th}^{*}$ and then its decay. For best fitting of the measured ER cross sections, with quasifission (qf) content, if any, the fusion-fission (ff) component is predicted. The magic-shell structure and entrance channel mass-asymmetry effects are analyzed, and the behavior of CN formation and survival probabilities ${P}_{\mathrm{CN}}$ and ${P}_{\mathrm{surv}}$ is studied.Methods: The quantum-mechanical fragmentation theory (QMFT) is used to predict the possible cold t-p combinations for synthesizing $^{220}\mathrm{Th}^{*}$, and the QMFT-based DCM is used to analyze its decay channels for the experimentally studied entrance channels. The only parameter of the model, the neck length $\mathrm{\ensuremath{\Delta}}R$, varies smoothly with the excitation energy ${E}^{*}$ of CN and is used to best fit the ER data and predict qf and ff cross sections.Results: The hot-compact and ``cold-elongated'' fragmentation paths show dissimilar results, whose comparisons with measured fission yields result in t-p combinations, the cold reaction valleys. For the decay process, the fixed $\mathrm{\ensuremath{\Delta}}R$ fit the measured ER cross section nicely, but not the individual decay-channel cross sections, which require the presence of qf effects, less so for asymmetric t-p combinations, and large (predicted) ff cross section.Conclusions: The calculated yields for hot-compact fragmentation path compared favorably with the observed asymmetric fission-mass distribution, resulting in mostly the same t-p reactions as are used in experiments. The best fitted $\mathrm{\ensuremath{\Delta}}R$ are found to be independent of the entrance channels, despite their very different cross sections, in conformity with our earlier works. The entrance-channel effects lead to the largest decay cross section for the most asymmetric and doubly magic t-p combination, the magicity taking over asymmetry for both the total ER and channel cross section. Furthermore, the variation of both ${P}_{\mathrm{CN}}$ and ${P}_{\mathrm{surv}}$ with ${E}^{*}$ fit in with the known systematic of other radioactive CN studied so far, thereby giving credence to our DCM analysis of $^{220}\mathrm{Th}^{*}$.

Electronic Polarization Effect of the Water Environment in Charge-Separated Donor-Acceptor Systems: An Effective Fragment Potential Model Study.

The electronic polarization (POL) of the surrounding environment plays a crucial role in the energetics of charge-separated systems. Here, the mechanism of POL in charge-separated systems is studied using a combined quantum mechanical and effective fragment potential (QM/EFP) method. In particular, the POL effect caused by charge separation (CS) is investigated at the atomic level by decomposition into the POL at each polarizability point. The relevance of the electric field generated by the CS is analyzed in detail. The model systems investigated are Na+-Cl- and guanine-thymine solvated in water. The dominant part of the POL arises from solvent molecules close to the donor (D) and acceptor (A) units. At short D-A distances, the electric field shows both positive and negative interferences. The former case enhances the POL energy. At longer distances, the interference is weakened, and the local electric field determines the POL energy.

Analysis of fission-fragment mass distribution within the quantum-mechanical fragmentation theory

The fission-fragment mass distribution is analysed for the 208Pb(18O, f) reaction within the quantum-mechanical fragmentation theory (QMFT). The reaction potential has been calculated by taking the binding energies, Coulomb potential and proximity potential of all possible decay channels and a stationary Schrodinger equation has been solved numerically to calculate the fission-fragment yield. The overall results for mass distribution are compared with those obtained in experiment. Fine structure dips in yield, corresponding to fragment shell closures at Z = 50 and N=82, which are observed by Bogachev et al., are reproduced successfully in the present calculations. These calculations will help to estimate the formation probabilities of fission fragments and to understand many related phenomena occurring in the fission process.

A Comprehensive Computational Study of the Interaction between Human Serum Albumin and Fullerenes

Human serum albumin (HSA) is the most abundant blood plasma protein, which transports fatty acids, hormones, and drugs. We consider nanoparticle-HSA interactions by investigating the binding of HSA with three fullerene analogs. Long MD simulations, quantum mechanical (fragment molecular orbital, energy decomposition analysis, atoms-in-molecules), and free energy methods elucidated the binding mechanism in these complexes. Such a systematic study is valuable due to the lack of comprehensive theoretical approaches to date. The main elements of the mechanism include the following: binding to IIA site results in allosteric modulation of the IIIA and heme binding sites with an increase in α-helical structure of IIIA. Fullerenes displayed high binding affinities for HSA; therefore, HSA can be used as a fullerene carrier, facilitating any toxic function the fullerene may exert. Complex formation is driven by hydrogen bonding, van der Waals, nonpolar, charge transfer, and dispersion energy contributions. Proper functionalization of C60 has enhanced its binding to HSA by more than an order of magnitude. This feature may be important for biological applications (e.g., photodynamic therapy of cancer). Satisfactory agreement with relevant experimental and theoretical data has been obtained.

α-decay chains of recoiled superheavy nuclei: A theoretical study

A systematic theoretical study of $\ensuremath{\alpha}$-decay half-lives in the superheavy mass region of the periodic table of elements is carried out by extending the quantum-mechanical fragmentation theory based on the preformed cluster model (PCM) to include temperature $(T)$ dependence in its built-in preformation and penetration probabilities of decay fragments. Earlier, the $\ensuremath{\alpha}$-decay chains of the isotopes of $Z=115$ were investigated by using the standard PCM for spontaneous decays, with``hot-optimum'' orientation effects included, which required a constant scaling factor of ${10}^{4}$ to approach the available experimental data. In the present approach of the PCM $(T\ensuremath{\ne}0)$, the temperature effects are included via the recoil energy of the residual superheavy nucleus (SHN) left after $x$-neutron emission from the superheavy compound nucleus. The important result is that the $\ensuremath{\alpha}$-decay half-lives calculated by the PCM $(T\ensuremath{\ne}0)$ match the experimental data nearly exactly, without using any scaling factor of the type used in the PCM. Note that the PCM $(T\ensuremath{\ne}0)$ is an equivalent of the dynamical cluster-decay model for heavy-ion collisions at angular momentum $\ensuremath{\ell}=0$. The only parameter of model is the neck-length parameter $\ensuremath{\Delta}R$, which for the calculated half-lives of $\ensuremath{\alpha}$-decay chains of various isotopes of $Z=113$ to 118 nuclei formed in ``hot-fusion'' reactions is found to be nearly constant, i.e., $\ensuremath{\Delta}R\ensuremath{\approx}0.95\ifmmode\pm\else\textpm\fi{}0.05$ fm for all the $\ensuremath{\alpha}$-decay chains studied. The use of recoiled residue nucleus as a secondary heavy-ion beam for nuclear reactions has also been suggested in the past.

Quantum mechanical fragment methods based on partitioning atoms or partitioning coordinates.

Conspectus The development of more efficient and more accurate ways to represent reactive potential energy surfaces is a requirement for extending the simulation of large systems to more complex systems, longer-time dynamical processes, and more complete statistical mechanical sampling. One way to treat large systems is by direct dynamics fragment methods. Another way is by fitting system-specific analytic potential energy functions with methods adapted to large systems. Here we consider both approaches. First we consider three fragment methods that allow a given monomer to appear in more than one fragment. The first two approaches are the electrostatically embedded many-body (EE-MB) expansion and the electrostatically embedded many-body expansion of the correlation energy (EE-MB-CE), which we have shown to yield quite accurate results even when one restricts the calculations to include only electrostatically embedded dimers. The third fragment method is the electrostatically embedded molecular tailoring approach (EE-MTA), which is more flexible than EE-MB and EE-MB-CE. We show that electrostatic embedding greatly improves the accuracy of these approaches compared with the original unembedded approaches. Quantum mechanical fragment methods share with combined quantum mechanical/molecular mechanical (QM/MM) methods the need to treat a quantum mechanical fragment in the presence of the rest of the system, which is especially challenging for those parts of the rest of the system that are close to the boundary of the quantum mechanical fragment. This is a delicate matter even for fragments that are not covalently bonded to the rest of the system, but it becomes even more difficult when the boundary of the quantum mechanical fragment cuts a bond. We have developed a suite of methods for more realistically treating interactions across such boundaries. These methods include redistributing and balancing the external partial atomic charges and the use of tuned fluorine atoms for capping dangling bonds, and we have shown that they can greatly improve the accuracy. Finally we present a new approach that goes beyond QM/MM by combining the convenience of molecular mechanics with the accuracy of fitting a potential function to electronic structure calculations on a specific system. To make the latter practical for systems with a large number of degrees of freedom, we developed a method to interpolate between local internal-coordinate fits to the potential energy. A key issue for the application to large systems is that rather than assigning the atoms or monomers to fragments, we assign the internal coordinates to reaction, secondary, and tertiary sets. Thus, we make a partition in coordinate space rather than atom space. Fits to the local dependence of the potential energy on tertiary coordinates are arrayed along a preselected reaction coordinate at a sequence of geometries called anchor points; the potential energy function is called an anchor points reactive potential. Electrostatically embedded fragment methods and the anchor points reactive potential, because they are based on treating an entire system by quantum mechanical electronic structure methods but are affordable for large and complex systems, have the potential to open new areas for accurate simulations where combined QM/MM methods are inadequate.

Dynamics of light, intermediate, heavy and superheavy nuclear systems formed in heavy-ion collisions

The dynamical description of light, intermediate, heavy and superheavy nuclei formed in heavy-ion collisions is worked out using the dynamical cluster decay model (DCM), with reference to various effects such as deformation and orientation, temperature, angular momentum etc. Based on the quantum mechanical fragmentation theory (QMFT), DCM has been applied to understand the decay mechanism of a large number of nuclei formed in low-energy heavy-ion reactions. Various features related to the dynamics of competing decay modes of nuclear systems are explored by addressing the experimental data of a number of reactions in light, intermediate, heavy and superheavy mass regions. The DCM, being a non-statistical description for the decay of a compound nucleus, treats light particles (LPs) or equivalently evaporation residues (ERs), intermediate mass fragments (IMFs) and fission fragments on equal footing and hence, provides an alternative to the available statistical model approaches to address fusion–fission and related phenomena.

Extensions of Natural Radioactivity to 4th-Type and of the Periodic Table to Super-heavy Nuclei: Contribution of Raj K Gupta to Cold Nuclear Phenomena

We have studied here the contribution of Indian Scientists associated with Prof. Raj K. Gupta to cold nuclear phenomena during the last almost four decades, which led to the discovery of fourth kind of natural radioactivity (also known as Cluster Radioactivity, CR) and to the extension of periodic table to super heavy nuclei. It is exclusively pointed out how the Quantum Mechanical Fragmentation Theory (QMFT) advanced by Prof. Raj K. Gupta and Collaborators led to the discovery of unique phenomenon of CR along with the predictions leading to the synthesis of super heavy elements. We have also mentioned the development of dynamical theories based on QMFT, the Preformed Cluster Model(PCM) and the dynamical cluster-decay model (DCM), to study the ground and excited state decays of nuclei, respectively, by Gupta and Collaborators. It is matter of great honor and pride for us to bring out this study to enthuse the young researchers to come up with novel ideas and have inspiration from the scientific contributions of Prof. Raj K. Gupta who is coincidentally celebrating his platinum jubilee birthday anniversary this year.

Synthesis of the doubly magic deformed nucleus108270Hs162in the decay of274Hs*formed via hot fusion reactions: Entrance-channel effects and role of magicity of48Ca and270Hs

Quantum mechanical fragmentation theory (QMFT) is used to look for all possible target-projectile (t,p) combinations forming the ``cold'' compound nucleus (CN) ${}^{274}$Hs${}^{*}$ at the CN excitation energy ${E}^{*}$ of ``hot, compact'' configuration. The predicted reactions, referring to potential energy minima, include all the three reactions ${}^{248}$Cm + ${}^{26}$Mg, ${}^{238}$U + ${}^{36}$S, and ${}^{226}$Ra + ${}^{48}$Ca already used in experiments, and a few more. The optimum ``cold'' and ``compact'' (t,p) combination, corresponding to lowest interaction barrier and smallest interaction radius, is one with largest mass asymmetry, but because of the doubly magic ${}^{48}$Ca nucleus, the evaporation residue cross sections for the ${}^{226}$Ra + ${}^{48}$Ca reaction are shown to be further enhanced. For the decay of CN ${}^{274}$Hs${}^{*}$, synthesizing ${}^{269--271}$Hs via 3$n$--5$n$ emission, we use the dynamical cluster-decay model (DCM) with effects of quadrupole deformations and ``hot'' compact orientations included in it, which support symmetric fission, in agreement with experiments. The fusion evaporation residue cross sections ${\ensuremath{\sigma}}_{xn}$, for $x=3$, 4, and 5 neutrons emission from the above-mentioned three entrance channels, are calculated within one parameter fitting, namely, the neck length. For best fitted neck-length parameter, the roles of entrance channel and that of magic shells are analyzed. In spite of different entrance channels resulting in different evaporation residue cross sections, the neck-length parameter at a given ${E}^{*}$ is shown to be independent of the entrance channel. The role of magic shells is shown in enhancing evaporation residue cross sections, not only for the entrance channel ${}^{226}$Ra + ${}^{48}$Ca, but also for the residue ${}^{270}$Hs, compared to its neighboring isotopes ${}^{269,271}$Hs. The fusion evaporation residue cross sections for the proposed new reactions, in synthesizing CN ${}^{274}$Hs${}^{*}$, are also estimated for future new experiments.

Tuned and Balanced Redistributed Charge Scheme for Combined Quantum Mechanical and Molecular Mechanical (QM/MM) Methods and Fragment Methods: Tuning Based on the CM5 Charge Model.

Tuned and balanced redistributed charge schemes have been developed for modeling the electrostatic fields of bonds that are cut by a quantum mechanical-molecular mechanical boundary in combined quantum mechanical and molecular mechanical (QM/MM) methods. First, the charge is balanced by adjusting the charge on the MM boundary atom to conserve the total charge of the entire QM/MM system. In the balanced smeared redistributed charge (BSRC) scheme, the adjusted MM boundary charge is smeared with a smearing width of 1.0 Å and is distributed in equal portions to the midpoints of the bonds between the MM boundary atom and the MM atoms bonded to it; in the balanced redistributed charge-2 (BRC2) scheme, the adjusted MM boundary charge is distributed as point charges in equal portions to the MM atoms that are bonded to the MM boundary atom. The QM subsystem is capped by a fluorine atom that is tuned to reproduce the sum of partial atomic charges of the uncapped portion of the QM subsystem. The new aspect of the present study is a new way to carry out the tuning process; in particular, the CM5 charge model, rather than the Mulliken population analysis applied in previous studies, is used for tuning the capping atom that terminates the dangling bond of the QM region. The mean unsigned error (MUE) of the QM/MM deprotonation energy for a 15-system test suite of deprotonation reactions is 2.3 kcal/mol for the tuned BSRC scheme (TBSRC) and 2.4 kcal/mol for the tuned BRC2 scheme (TBRC2). As was the case for the original tuning method based on Mulliken charges, the new tuning method performs much better than using conventional hydrogen link atoms, which have an MUE on this test set of about 7 kcal/mol. However, the new scheme eliminates the need to use small basis sets, which can be problematic, and it allows one to be more consistent by tuning the parameters with whatever basis set is appropriate for applications. (Alternatively, since the tuning parameters and partial charges obtained by the new method do not depend strongly on basis set, one can continue to use available CM5-tuned parameters even when one changes the basis set.) Furthermore, we found that, as compared to Mulliken charges, the CM5 charges describe the charge distributions in test molecules better, and they reproduce the dipole moments of full quantum mechanical calculations better; therefore the new tuning procedure is more physical and should be more reliable and robust.

Clusters in 18,20O and 22Ne nuclei using the quantum mechanical fragmentation theory

We examine for the first time the possibility of 14C clustering, in the resonant excited states of 18,20O and 22Ne nuclei, on the basis of the quantum mechanical fragmentation theory (QMFT). The important point is to include appropriate pairing strength δ in the temperature-dependent liquid drop energy.