Quantum Molecular Dynamics Approach Research Articles

The Coordination of Uranyl in Water: A Combined Quantum Chemical and Molecular Simulation Study

The coordination environment of uranyl in water has been studied using a combined quantum mechanical and molecular dynamics approach. Multiconfigurational wave function calculations have been performed to generate pair potentials between uranyl and water. The quantum chemically determined energies have been used to fit parameters in a polarizable force field with an added charge transfer term. Molecular dynamics simulations have been performed for the uranyl ion and up to 400 water molecules. The results show a uranyl ion with five water molecules coordinated in the equatorial plane. The U-O(H(2)O) distance is 2.40 A, which is close to the experimental estimates. A second coordination shell starts at about 4.7 A from the uranium atom. No hydrogen bonding is found between the uranyl oxygens and water. Exchange of waters between the first and second solvation shell is found to occur through a path intermediate between association and interchange. This is the first fully ab initio determination of the solvation of the uranyl ion in water.

Accurate and efficient non-adiabatic quantum molecular dynamics approach for laser–matter interactions

A non-adiabatic quantum molecular dynamics approach for treating the interaction of matter with intense, short-duration laser pulses is developed. This approach, which is parallelized to run on massively-parallel supercomputers, is shown to be both accurate and efficient. Illustrative results are presented for harmonic generation occurring in diatomic molecules using linearly polarized laser pulses.

Quantum molecular dynamics approach to estimate spallation yield fromp +208Pb reaction at 800 MeV

The spallation yield of neutrons and other mass fragments produced in 800 MeV proton induced reaction on208Pb have been calculated in the framework of quantum molecular dynamics (QMD) model. The energy spectra and angular distribution have been calculated. Also, multiplicity distributions of the emitted neutrons and kinetic energy carried away by them have been estimated and compared with the available experimental data. The agreement is satisfactory. A major contribution to the neutron emission comes from statistical decay of the fragments. For mass and charge distributions of spallation products the QMD process gives rise to target-like and projectile-like fragments only.

HBT Parameters and Space-Momentum Correlationsin Relativistic Heavy-Ion Collisions

Using the relativistic quantum molecular dynamics approach,with a correlation after-burner,the physics of the transverse momentum dependenceof the Hanbury-Brown and Twiss parametersis studied for Au + Au, Si + Si and p + p collisionsat the centre-of-mass energy (s)1/2 = 200 AGeV.The results indicate that the space-momentum correlations wouldaffect such dependence in both heavy-ion and elementary collisions.The size parameters as a function of the transverse mass mtare sensitive to the degree of space-momentum correlations.

Multifragmentation – what the data tell us about the different models

We discuss what the presently collected data tell us about the mechanism of multifragmentation by comparing the results of two different models, which assume or show an opposite reaction scenario, with the recent high statistics $4\pi$ experiments performed by the INDRA collaboration. We find that the statistical multifragmentation model and the dynamical Quantum Molecular Dynamics approach produce almost the same results and agree both quite well with experiment. We discuss which observables may serve to overcome this deadlock on the quest for the reaction mechanism. Finally we proof that even if the system is in equilibrium, the fluctuation of the temperature due to the smallness of the system renders the caloric curve useless for the proof of a first order phase transition.

Laser driven ground state photodissociation of HCo(CO)4: An ab initio quantum molecular dynamics approach

The competition between laser selective dissociation of the axial Co–CO bond and intramolecular energy redistribution in HCo(CO)4 is investigated. Our approach is based on an ab initio calculation of the Cartesian reaction surface Hamiltonian along the Co–CO bond distance using density functional theory. The substrate degrees of freedom are treated within the harmonic approximation. We find only two modes which are strongly coupled to the dissociation reaction, one which can be approximately described as an H–Co stretching vibration while the second one is an umbrella type vibration of the equatorial carbonyls. The region of validity for harmonic motion of the substrate normal modes is explored.

Relativistic hadron-hadron collisions in the ultra-relativistic quantum molecular dynamics model

Hadron-hadron (h-h) collisions at high energies are investigated in the ultra-relativistic quantum molecular dynamics (UrQMD) approach. This microscopic transport model describes the phenomenology of hadronic interactions at low and intermediate energies (<5 GeV) in terms of interactions between known hadrons and their resonances. At higher energies, >5 GeV, the excitation of colour strings and their subsequent fragmentation into hadrons dominates the multiple production of particles in the UrQMD model. The model shows a fair overall agreement with a large body of experimental h-h data over a wide range of h-h centre-of-mass energies. Hadronic reaction data with higher precision would be useful to support the use of the UrQMD model for relativistic heavy-ion collisions.

Evidence of Early Multistrange Hadron Freeze-Out in High Energy Nuclear Collisions

Recently reported transverse momentum distributions of strange hadrons produced in Pb(158AGeV) on Pb collisions and corresponding results from the relativistic quantum molecular dynamics (RQMD) approach are examined. We argue that the experimental observations favor a scenario in which multi-strange hadrons are formed and decouple from the system rather early at large energy densities (around 1 GeV/fm$^3$). The systematics of the strange and non-strange particle spectra indicate that the observed transverse flow develops mainly in the late hadronic stages of these reactions.

Quantum molecular dynamics approach to the nuclear matter below the saturation density

Quantum molecular dynamics is applied to study the ground state properties of nuclear matter at subsaturation densities. Clustering effects are observed to soften the equation of state at these densities. The structure of nuclear matter at subsaturation density shows some exotic shapes with variation of the density.

In-medium dependence and Coulomb effects of the pion production in heavy ion collisions

The properties of the high energy pions observed in heavy ion collisions, in particular in the system Au on Au at 1 GeV/nucleon are investigated. The reaction dynamics is described within the Quantum Molecular Dynamics (QMD) approach. It is shown that high energy pions freeze out early and originate from the hot, compressed matter. N*-resonances are found to give an important contribution toward the high energy tail of the pion. Further the role of in-medium effects in the description of charged pion yields and spectra is investigated using a microscopic potential derived from the Brueckner G-matrix which is obtained with the Reid soft-core potential. It is seen that the high energy part of the spectra is relatively more suppressed due to in-medium effects as compared to the low energy part. A comparison to experiments further demonstrates that the present calculations describe reasonably well the neutral (TAPS) and charged (FOPI) pion spectra. The observed energy dependence of the π−/π+ ratio, i.e. deviations from the isobar model prediction, is due to Coulomb effects and again indicate that high energy pions probe the hot and dense phase of the reaction. These findings are confirmed independently by a simple phase space analysis.

Analysis of proton-induced fragment production cross sections by the quantum molecular dynamics plus statistical decay model.

The production cross sections of various fragments from proton-induced reactions on $^{56}$Fe and $^{27}$Al have been analyzed by the Quantum Molecular Dynamics (QMD) plus Statistical Decay Model (SDM). It was found that the mass and charge distributions calculated with and without the statistical decay have very different shapes. These results also depend strongly on the impact parameter, showing an importance of the dynamical treatment as realized by the QMD approach. The calculated results were compared with experimental data in the energy region from 50 MeV to 5 GeV. The QMD+SDM calculation could reproduce the production cross sections of the light clusters and intermediate-mass to heavy fragments in a good accuracy. The production cross section of $^{7}$Be was, however, underpredicted by approximately 2 orders of magnitude, showing the necessity of another reaction mechanism not taken into account in the present model.

Disappearance of Collective Rotation in Heavy Ion Collisions11The project supported partly by National Natural Science Foundation of China

A theoretical analysis of collective rotation is performed in heavy ion collisions below 100 MeV/nucleon in the quantum molecular dynamics approach. Both methods are contributed to this analysis, one is based on the shapes of azimuthal distribution, and the other is based on the numerical semiclassical calculation of rotation. The collective rotation becomes weaker with the increasing of beam energy, and tends to fade out at a certain beam energy. The impact parameter dependence of collective motion is also discussed. In connection with recent experiments, theoretical results and experimental data are compared by taking into account the fluctuation of experimental reaction plane determination.

Phasespace structure of antideuteron production in heavy ion collisions

In the framework of the relativistic quantum molecular dynamics approach (RQMD) we investigate antideuteron ( d ) observables in Au+Au collisions at 10.7 AGeV. The impact parameter dependence of the formation rates d p 2 and d p 2 is calculated. In central collisions, the antideuteron formation rate is predicted to be two orders of magnitude lower than the deuteron formation rate. The d yield in central Au+Au collisions is one order of magnitude lower than in Si+Al collisions. In semicentral collisions different configuration space distributions of p' s and d' s lead to a large “squeeze-out” effect for antideuterons, which is not predicted for the p' s .

Correlation functions and the disappearance of rotational collective motion in nucleus-nucleus collisions below 100 MeV/nucleon.

The azimuthal correlation function has been calculated as a function of beam energy and impact parameter for the $^{40}\mathrm{Ar}$${+}^{27}$Al system from 36 MeV/nucleon to 100 MeV/nucleon via a quantum molecular dynamics (QMD) approach. The anisotropy of the azimuthal correlation at midrapidity interval becomes weaker with increasing beam energy and collision centrality. Around 100 MeV/nucleon the azimuthal correlation tends to be flat which indicates the rotational collective motion disappears. The vanishing of rotation may be due to the short lifetime of the emitter as suggested by the two-particle correlation function at small relative momenta. A method to estimate the fluctuation of the experimental reaction plane with respect to the true reaction plane is presented also via the azimuthal correlation function.

Thermalization effects in heavy-ion collisions

Simulation of heavy-ion (HI) collisions is performed within the quantum molecular dynamics (QMD) approach to investigate the thermalization process in 40Ca+ 40Ca collisions at energies up to 400 MeV/u. The results of the QMD simulation are analyzed within a hot Thomas-Fermi model to study the entropy evolution during these collisions. An anomalous decrease of the total one-body entropy at the decompression phase is found, which might be connected to a dynamical instability in the decompressed HI system. Here the many-body correlations are expected to be strong, leading to a break-down of the Fermi-gas-model description of the thermalization process.

When and how are fragments formed in heavy ion collisions?

Employing the Quantum Molecular Dynamics approach for simulating the fragment formation in nucleus nucleus collisions we test the predictive power of the recently advanced Early Cluster Recognition Algorithm. We find that this algorithm gives almost the same number of fragments as the standard minimum spanning tree procedure. It is applicable not only to the late stage of the reaction. Taking advantage of this fact we find that the final fragmentation pattern is already realized shortly after the initial phase of violent collisions between the projectile and target nucleons, long before they are supposed to be formed in thermal models.

"Antiflow" of antiprotons in heavy ion collisions.

In the framework of the relativistic quantum molecular dynamics approach we investigate antiproton (p\ifmmode\bar\else\textasciimacron\fi{}) observables in Au+Au collisions at 10.7A GeV. The rapidity dependence of the in-plane directed transverse momentum ${\mathbf{p}}_{\mathit{x}}$(y) of p\ifmmode\bar\else\textasciimacron\fi{}'s shows the opposite sign of the nucleon flow, which has indeed recently been discovered at 10.7A GeV by the E877 group. The ``antiflow'' of p\ifmmode\bar\else\textasciimacron\fi{}'s is also predicted at 2A GeV and at 160A GeV and appears at all energies also for \ensuremath{\pi}'s and ${\mathit{K}}^{\mathrm{\ensuremath{-}}}$'s. These predicted p\ifmmode\bar\else\textasciimacron\fi{} anticorrelations are a direct proof of strong p\ifmmode\bar\else\textasciimacron\fi{} annihilation in massive heavy ion reactions.

Rapidity dependence of soft mesons as a signature of resonance matter production in nucleus-nucleus collisions.

Enhanced production of soft---i.e., low transverse momentum---pions has been experimentally observed in nucleus-nucleus collisions at beam energies of 14.5{ital A} GeV [the Brookhaven National Laboratory Alternating Gradient Synchrotron (AGS)] and 200{ital A} GeV (CERN Super Proton Synchrotron). Here the rapidity dependence of soft pion and kaon production for the reaction Si(14.5{ital A} GeV)+Au is studied in the framework of the relativistic quantum molecular dynamics (RQMD) approach. The softest mesons from decaying resonance matter, which turns out to be the main production mechanism in RQMD, show a peculiar rapidity dependence, a weakening of the signal in the central rapidity region of the participant nucleons. This is in striking contrast to expectations from all other suggested mechanisms like mean fields, collective flow, and quantum statistical effects. The prediction can be tested by forthcoming experiments at the AGS in the near future. It is also predicted that a similar, though quantitatively much weaker, signal can be observed in the kaon spectra.

Computer simulation of materials using parallel architectures

Algorithms are designed to implement molecular dynamics (MD) and quantum molecular dynamics (QMD) simulations on emerging concurrent architectures. For systems with finite-range interactions, a domain-decomposition algorithm is used to implement the multiple-time-step (MTS) approach to MD simulations on distributed-memory multiple instruction multiple data (MIMD) machines. Parallel algorithms are also designed for MD simulations of bulk Coulombic systems. The performances of these algorithms are tested on the Intel iPSC/860 system. The computational complexities of these algorithms are O( N) and parallel efficiencies close to 0.9. The core computational kernel of the QMD approach consists of solutions of parabolic partial differential equations (PDE) such as the time-dependent Schrödinger equation or time-dependent Kohn-Sham equation. This problem is coupled with another computationally intensive problem, i.e., solution of elliptic PDEs (the Poisson equation) for the long-range electron-electron interaction. We have designed parallel algorithms for both problems on SIMD (single instruction multiple data) machines. In the past two years, we have used the parallel computer architectures in our Concurrent Computing Laboratory for Materials Simulations (CCLMS) to carry out MD and QMD simulations on network glasses, ceramic composites, nanophase materials, solid C 60 and graphitic tubules, and quantum transport in nanoscale devices. Structural transformation, intermediate-range order and dynamic behavior of SiO 2 glass at high pressures are investigated with molecular dynamics. At high densities, the height of the first sharp diffraction peak is considerably diminished, its position changes from 1.6 to 2.2 Å −1, and a new peak appears at 2.85 Å -1. At twice the normal density, SiO bond length increases, SiO coordination changes from 4 to 6 and OSiO bond-angle changes from 109° to 90°. This is a tetrahedral to octahedral transformation, which was reported recently by Meade, Hem1ey, and Mao. Molecular dynamics simulations of porous silica, in the density range 2.2-0.1 g/cm 3, are carried out on a 41 472 particle system using a MIMD computer. The internal surface area, pore surface-to-volume ratio, pore-size distribution, fractal dimension, correlation length and mean-particle size are determined as a function of the density. Structural transition between a condensed amorphous phase and a low-density porous phase is characterized by these quantities. Various dissimilar porous structures with different fractal dimensions are obtained by controlling the preparation schedule and temperature.

Quantum molecular dynamics simulation of multifragment production in heavy ion collisions at E/A=600 MeV.

With the ALADIN forward spectrometer the fragmentation of gold nuclei at 600 MeV per nucleon after interaction with carbon, aluminum, copper, and lead targets has been investigated. The results are compared to quantum-molecular-dynamics (QMD) calculations using soft and hard equations of state as well as a soft equation of state with momentum-dependent forces. Whereas the QMD has been successfully applied to heavy ion collisions at lower energies, it is not possible to reproduce the fragment distributions and the light-particle multiplicities observed in this experiment at relativistic energies. To study the reasons for the discrepancy between the experimental data and the simulations we investigated the time evolution of the nuclear system after a collision and the disintegration pattern of excited nuclei in the QMD approach.