Full Quantum Mechanical Calculations Research Articles

Conventional and Unconventional Dicke Models: Multistabilities and Nonequilibrium Dynamics.

The Dicke model describes the collective behavior of a subwavelength-size ensemble of two-level atoms (i.e., spin-1/2) interacting identically with a single quantized radiation field of a cavity. Across a critical coupling strength it exhibits a zero-temperature phase transition from the normal state to the superradiant phase where the field is populated and the collective spin acquires a nonzero x component, which can be imagined as ferromagnetic ordering of the atomic spins along x. Here we introduce a variant of this model where two subwavelength-size ensembles of spins interact with a single quantized radiation field with different strengths. Subsequently, we restrict ourselves to a special case where the coupling strengths are opposite (which is unitarily equivalent to equal-coupling strengths). Because of the conservation of the total spin in each ensemble individually, the system supports two distinct superradiant states with x-ferromagnetic and x-ferrimagnetic spin ordering, coexisting with each other in a large parameter regime. The stability and dynamics of the system in the thermodynamic limit are examined using a semiclassical approach, which predicts nonstationary behaviors due to the multistabilities. At the end, we also perform small-scale full quantum-mechanical calculations, with results consistent with the semiclassical ones.

The NUMEN project: probing nuclear response to weak interaction by nuclear reactions

Different reactions channels induced by the 18O + 40Ca collisions at 275 MeV incident energy are simultaneously measured and analysed consistently within the same reaction and structure frameworks within the NUMEN project. The project aims to provide data-driven information for the determination of the nuclear matrix elements involved in the neutrinoless double beta decay. In particular, the elastic and inelastic scattering, one- and two-proton transfer, one-neutron transfer, and single charge exchange reactions are explored. The full quantum-mechanical calculations, performed by including microscopic nuclear structure inputs, describe well all the experimental data, giving support to a multi-channel strategy for the analysis of heavy-ion induced direct reactions.

Scattered spectra from inverse Compton sources operating at high laser fields and high electron energies

As Compton x-ray and gamma-ray sources become more prevalent, to understand their performance in a precise way, it becomes important to be able to compute the distribution of scattered photons precisely. For example, codes have been developed at Old Dominion University which were used to understand the performance of the Dresden Compton Source in detail. An ideal model would (i) include the full Compton effect frequency relations between incident and scattered photons, (ii) allow the field strength to be large enough that nonlinear effects are captured, and (iii) allow the effects of electron beam emittance to be introduced and studied. Various authors have considered various pieces of this problem, but until now, no analytical or numerical procedure is known to us that captures these three effects simultaneously. Here we present a model for spectrum calculations which simultaneously cover these aspects. The model is compared to a published full quantum mechanical calculation and found to agree for a case where both full Compton effect and nonlinear field strength are present. We use this model to investigate chirping prescriptions to mitigate ponderomotive broadening.

Electron transmission and mean free path in molybdenum disulfide at electron-volt energies

In van der Waals (vdW) materials, the electron mean free path (MFP) is largely influenced by the discrete states in the unoccupied band structure. So far, the influence of these states has only been measured in graphene, while all measurements on other vdW materials lack energy resolution. Here, we present reflection and transmission spectra of freestanding, few-layered molybdenum disulfide $(\mathrm{Mo}{\mathrm{S}}_{2})$ samples in the $0--55$ eV electron range. Our measurements reveal states of enhanced electron transmissivity above the vacuum level, that correspond to the (unoccupied) density of states. We also show a full quantum-mechanical calculation that confirms a good understanding of the elastic scattering in $\mathrm{Mo}{\mathrm{S}}_{2}$. A model is developed to extract the inelastic MFP spectrum, which is a measure of the inelastic scattering cross section. As $\mathrm{Mo}{\mathrm{S}}_{2}$ is a complicated system of different atomic planes, we expect that our methods generalize well to other van der Waals materials and heterostacks thereof.

Study of Fusion Reactions of some Light projectiles on Medium Targets

Background and objectives: The investigation of heavy-ion (HI) induced fusion processes in order to comprehend the many mechanisms involved in these reactions has long been a focus of nuclear physics. The complicated structure and behavior of projectile and target nuclei with various projectile energy allows us to define the reaction process and may aid in the investigation of the potential of creating superheavy elements (SHE) in the laboratory. Methods: The semiclassical and full quantum mechanical complete fusion cross section calculations and the distribution of the fusion barrier for the systems 12C+50Ti,15N+56Fe,16O+63Cu and 40Ar+27Al. The calculations have been carried out by using SCF code for semiclassical and CC code for quantum mechanical calculations. To account for the target and projectile relative motion, we modified these codes. The fusion barrier distribution to be calculated directly from this code by subroutines written in Fortran language and added to the main code. The coupled-channel is considered by taking two channels only due to the difficulty of solving coupled differential equations. Results: The obtained results for and the for the studied systems were compared to the related measured data and discussed. Conclusion: The semiclassical method used to describe the fusion reaction agrees very well with the quantum method and with the measured data and we can conclude that this semiclassical method can be used as alternative to quantum method to study the fusion reaction.

Machine Learning for Fast, Quantum Mechanics-Based Approximation of Drug Lipophilicity.

Lipophilicity, as measured by the partition coefficient between octanol and water (log P), is a key parameter in early drug discovery research. However, measuring log P experimentally is difficult for specific compounds and log P ranges. The resulting lack of reliable experimental data impedes development of accurate in silico models for such compounds. In certain discovery projects at Novartis focused on such compounds, a quantum mechanics (QM)-based tool for log P estimation has emerged as a valuable supplement to experimental measurements and as a preferred alternative to existing empirical models. However, this QM-based approach incurs a substantial computational cost, limiting its applicability to small series and prohibiting quick, interactive ideation. This work explores a set of machine learning models (Random Forest, Lasso, XGBoost, Chemprop, and Chemprop3D) to learn calculated log P values on both a public data set and an in-house data set to obtain a computationally affordable, QM-based estimation of drug lipophilicity. The message-passing neural network model Chemprop emerged as the best performing model with mean absolute errors of 0.44 and 0.34 log units for scaffold split test sets of the public and in-house data sets, respectively. Analysis of learning curves suggests that a further decrease in the test set error can be achieved by increasing the training set size. While models directly trained on experimental data perform better at approximating experimentally determined log P values than models trained on calculated values, we discuss the potential advantages of using calculated log P values going beyond the limits of experimental quantitation. We analyze the impact of the data set splitting strategy and gain insights into model failure modes. Potential use cases for the presented models include pre-screening of large compound collections and prioritization of compounds for full QM calculations.

Complex collisions of ultracold molecules: A toy model

We introduce a model to study the collisions of two ultracold diatomic molecules in one dimension interacting via pairwise potentials. We present results for this system and argue that it offers lessons for real molecular collisions in three dimensions. We analyze the distribution of the adiabatic potentials in the hyperspherical coordinate representation as well as the distribution of near-threshold four-body bound states, systematically studying the effects of molecular properties, such as interaction strength, interaction range, and atomic mass. It is found that the adiabatic potential's nearest-neighbor energy level distribution transitions from significant level repulsion characteristic of chaos (Brody distribution) to nonchaotic (Poisson distribution) as the two molecules are separated. For the near-threshold four-atom bound states, the case where all atoms have equal masses shows a Poissonian spacing distribution, while the unequal-mass system exhibits significant level repulsion characterized by a nonzero Brody parameter. We derive a semiclassical formula for the density of states and extract from it simple scaling laws with potential depth and range. We find good agreement between the semiclassical predictions for the density of states and the full quantum mechanical calculations.

Machine learning builds full-QM precision protein force fields in seconds.

Full-quantum mechanics (QM) calculations are extraordinarily precise but difficult to apply to large systems, such as biomolecules. Motivated by the massive demand for efficient calculations for large systems at the full-QM level and by the significant advances in machine learning, we have designed a neural network-based two-body molecular fractionation with conjugate caps (NN-TMFCC) approach to accelerate the energy and atomic force calculations of proteins. The results show very high precision for the proposed NN potential energy surface models of residue-based fragments, with energy root-mean-squared errors (RMSEs) less than 1.0kcal/mol and force RMSEs less than 1.3kcal/mol/Å for both training and testing sets. The proposed NN-TMFCC method calculates the energies and atomic forces of 15 representative proteins with full-QM precision in 10-100s, which is thousands of times faster than the full-QM calculations. The computational complexity of the NN-TMFCC method is independent of the protein size and only depends on the number of residue species, which makes this method particularly suitable for rapid prediction of large systems with tens of thousands or even hundreds of thousands of times acceleration. This highly precise and efficient NN-TMFCC approach exhibits considerable potential for performing energy and force calculations, structure predictions and molecular dynamics simulations of proteins with full-QM precision.

A Constrained Analysis of the 40Ca(18O,18F)40K Direct Charge Exchange Reaction Mechanism at 275 MeV

The40Ca(18O,18F)40K single charge exchange (SCE) reaction is explored at an incident energy of 275 MeV and analyzed consistently by collecting the elastic scattering and inelastic scattering data under the same experimental conditions. Full quantum-mechanical SCE calculations of the direct mechanism are performed by including microscopic nuclear structure inputs and adopting either a bare optical potential or a coupled channel equivalent polarization potential (CCEP) constrained by the elastic and inelastic data. The direct SCE mechanism describes the magnitude and shape of the angular distributions rather well, thus suggesting the suppression of sequential multi-nucleon transfer processes.

Towards a complete elucidation of the ro-vibrational band structure in the SF6 infrared spectrum from full quantum-mechanical calculations.

The first accurate and complete theoretical room-temperature rotationally resolved spectra in the range 300-3000 cm-1 are reported for the three most abundant isotopologues (32SF6, 33SF6 and 34SF6) of the sulfur hexafluoride molecule. The literature reports that SF6 is widely used as a prototype molecule for studying the multi-photon excitation processes with powerful lasers in the infrared range. On the other hand, SF6 is an important greenhouse molecule with a very long lifetime in the atmosphere. Because of relatively low vibrational frequencies, the hot bands of this molecule contribute significantly to the absorption infrared spectra even at room temperature. This makes the calculation of complete rovibrational line lists required for fully converged opacity modeling extremely demanding. In order to reduce the computational costs, symmetry was exploited at all stages of the first global variational nuclear motion calculations by means of irreducible tensor operators. More than 2600 new vibrational band centers were predicted using our empirically refined ab initio potential energy surface. Highly excited rotational states were calculated up to J = 121, resulting in 6 billion transitions computed from an ab initio dipole moment surface and distributed over more than 500 cold and hot bands. The final line lists are made available through the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru). For the first time, the major (ro)vibrational band structures in the wavenumber range corresponding to the strongest absorption in the infra-red are completely elucidated for a seven-atom molecule, providing excellent agreement with the observed spectral patterns. It is shown that the obtained results are more complete than all available line lists, permitting reliable modelling of the temperature dependence of the molecular opacity.

Comparison of semiclassical transfer to continuum model with Ichimura-Austern-Vincent model in medium energy knockout reactions

The full quantum mechanical (QM) model of inclusive breakup of Ichimura-Austern-Vincent (IAV) is implemented in this paper to calculate breakup from heavy radioactive nuclei on a 9Be target at intermediate energies. So far it had been implemented and applied only to low energy reactions with light projectiles. The IAV model is successful in predicting absolute cross sections among other observables. In order to get insight on the content of the model in the case of the complicated heavy-ion reactions, results are compared with those of the semiclassical transfer to the continuum (TC) model. Because the TC is based on analytical formulae the dynamics of the breakup as it is contained in the rather involved IAV formalism will become more transparent. Heavy-ion reactions at high energies (>50 A.MeV) are demanding from the computational point of view because of the high number of partial waves involved, typically around 100. The TC constitutes a useful alternative to the full QM calculations whenever predictions and/or estimates are necessary. It allows also for a systematic, fast evaluation of breakup observables. In the applications of both methods we use state-of-the art optical potentials and structure information. Excellent agreement is found between the calculated results of both methods and with available experimental data which shows that the qualitative and quantitative understanding of most aspects of one nucleon breakup is well under control.

Predi-XY: a python program for automated generation of NICS-XY-scans based on an additivity scheme

Polycyclic aromatic systems are prevalent in chemistry and materials science because their thermodynamic stability, planarity, and tunable electronic properties make them uniquely suited for various uses. These properties are closely linked to the aromaticity of the systems. Therefore, characterizing the aromatic behavior is useful for designing new functional compounds and understanding their reactivity. NICS-XY-scans are a popular and simple tool for investigating the aromatic trends in polycyclic systems. Herein we present Predi-XY: an automated system for generating NICS-XY-scans for polycyclic aromatic systems using an additivity scheme. The program provides the predicted scans at a fraction of the computational cost of a full quantum mechanical calculation and enables rapid comparison of various polycyclic aromatic systems.

Two-dimensional Raman spectroscopy of Lennard-Jones liquids via ring-polymer molecular dynamics.

The simulation of multidimensional vibrational spectroscopy of condensed-phase systems including nuclear quantum effects is challenging since full quantum-mechanical calculations are still intractable for large systems comprising many degrees of freedom. Here, we apply the recently developed double Kubo transform (DKT) methodology in combination with ring-polymer molecular dynamics (RPMD) for evaluating multi-time correlation functions [K. A. Jung et al., J. Chem. Phys. 148, 244105 (2018)], providing a practical method for incorporating nuclear quantum effects in nonlinear spectroscopy of condensed-phase systems. We showcase the DKT approach in the simulation of the fifth-order two-dimensional (2D) Raman spectroscopy of Lennard-Jones liquids as a prototypical example, which involves nontrivial nonlinear spectroscopic observables of systems described by anharmonic potentials. Our results show that the DKT can faithfully reproduce the 2D Raman response of liquid xenon at high temperatures, where the system behaves classically. In contrast, liquid neon at low temperatures exhibits moderate but discernible nuclear quantum effects in the 2D Raman response compared to the responses obtained with classical molecular dynamics approaches. Thus, the DKT formalism in combination with RPMD simulations enables simulations of multidimensional optical spectroscopy of condensed-phase systems that partially account for nuclear quantum effects.

Hybrid Combination of Quantum Mechanics with Quantum-Based Polarizable Reactive Force Field for Large Scale Full Solvent Simulations of Electrocatalysis

To develop new generations of electrocatalysts required for energy and environmental sustainability, we need the accuracy of full solvent quantum mechanics (QM) (free energy barriers to 0.05 eV, onset potentials to 0.05 V) but for practical sized nanoparticles and catalysts (1000’s to millions of atoms). We report here a solution to this problem. We start with the RexPoN reactive force field that provides higher accuracy than density functional theory (DFT) and combine it with QM to accurately include long-range interactions and polarization effects to enable reactive simulations with QM accuracy in the presence of solvent including 1000’s to millions of waters. Here we apply this RexPoN embedded QM (ReQM) to reactive simulations of electrocatalysis demonstrating that ReQM accurately replaces DFT water for computing the Raman frequencies of reaction intermediates during CO2 reduction to ethylene, with comparisons to operando electrocatalysis experiments and to full solvent QM calculations.

An On-the-Fly Approach to Construct Generalized Energy-Based Fragmentation Machine Learning Force Fields of Complex Systems.

An on-the-fly fragment-based machine learning (ML) approach was developed to construct machine learning force fields for large complex systems. In this approach, the energy, forces, and molecular properties of the target system are obtained by combining machine learning force fields of various subsystems with the generalized energy-based fragmentation (GEBF) approach. Using a nonparametric Gaussian process (GP) model, all the force fields of subsystems are automatically generated online without data selection and parameter optimization. With the GEBF-ML force field constructed for a normal alkane, C60H122, long-time molecular dynamics (MD) simulations are performed on different sizes of alkanes, and the predicted energy, forces, and molecular properties (dipole moment) are favorably comparable with full quantum mechanics (QM) calculations. The predicted IR spectra also show excellent agreement with the direct ab initio MD results. Our results demonstrate that the GEBF-ML method provides an automatic and efficient way to build force fields for a broad range of complex systems such as biomolecules and supramolecular systems.

Quantitative Prediction of Aggregation-Induced Emission: A Full Quantum Mechanical Approach to the Optical Spectra.

Full quantum mechanical (FQM) calculation of the excited state of aggregation-induced-emission (AIE) materials is highly sought but still a challenging task. Herein, we employed the recently developed electrostatically embedded generalized molecular fractionation (EE-GMF) method, a method based on the systematic fragmentation approach, to predict, for the first time, the spectra of a prototype AIE fluorophore: di(p-methoxylphenyl)dibenzofulvene (FTPE). Compared to the single molecular or QM/MM calculations, the EE-GMF method shows significantly improved accuracy, nearly reproducing the experimental optical spectra of FTPE in both condensed phases. Importantly, we show that the conventional restriction of the intramolecular rotation mechanism cannot fully account for AIE, whereas the two-body intermolecular quantum mechanical interaction plays a crucial role in AIE.

Computation of Electromagnetic Properties of Molecular Ensembles.

We outline a methodology for efficiently computing the electromagnetic response of molecular ensembles. The methodology is based on the link that we establish between quantum‐chemical simulations and the transfer matrix (T‐matrix) approach, a common tool in physics and engineering. We exemplify and analyze the accuracy of the methodology by using the time‐dependent Hartree‐Fock theory simulation data of a single chiral molecule to compute the T‐matrix of a cross‐like arrangement of four copies of the molecule, and then computing the circular dichroism of the cross. The results are in very good agreement with full quantum‐mechanical calculations on the cross. Importantly, the choice of computing circular dichroism is arbitrary: Any kind of electromagnetic response of an object can be computed from its T‐matrix. We also show, by means of another example, how the methodology can be used to predict experimental measurements on a molecular material of macroscopic dimensions. This is possible because, once the T‐matrices of the individual components of an ensemble are known, the electromagnetic response of the ensemble can be efficiently computed. This holds for arbitrary arrangements of a large number of molecules, as well as for periodic or aperiodic molecular arrays. We identify areas of research for further improving the accuracy of the method, as well as new fundamental and technological research avenues based on the use of the T‐matrices of molecules and molecular ensembles for quantifying their degrees of symmetry breaking. We provide T‐matrix‐based formulas for computing traditional chiro‐optical properties like (oriented) circular dichroism, and also for quantifying electromagnetic duality and electromagnetic chirality. The formulas are valid for light‐matter interactions of arbitrarily‐high multipolar orders.

Theoretical investigation of the pressure broadening D1 and D2 lines of cesium atoms colliding with ground-state helium atoms

Full quantum mechanical calculations are performed to determine the broadening in the far wings of the cesium D1 and D2 line shapes arising from elastic collisions of Cs atom with inert helium atoms. The potential energy curves of the low-lying CsHe molecular states, as well as the related transition dipole moments, are carefully computed from ab initio methods based on state-averaged complete active space self-consistent field–multireference configuration interaction (SA-CASSCF–MRCI) calculations, involving the spin–orbit effect, and taking into account the Davidson and BSSE corrections. The absorption and emission reduced coefficients are determined in the temperature and wavelength ranges of 323–3000 K and 800–1000 nm, respectively. Both profiles of the absorption and the emission are dominated by the free–free transitions, and exhibit a satellite peak in the blue wing near the wavelength 825 nm, attributed to transitions. The results are in good agreement with previous experimental and theoretical works.

Full dimensional quantum mechanical calculations of the reaction probability of the H + NH3 collision based on a mixed Jacobi and Radau description.

The collision between hydrogen and ammonia is a benchmark system to study chemical elementary reactions with five atoms. In this work, we present a description of the system based on mixed Jacobi and Radau coordinates combined with the time-dependent wave packet method to study the H + NH3 reaction. The Radau coordinates are used to describe the reactive moiety NH2. A salient feature of this approach is that the present coordinates have a great advantage that a very small number of basis set functions can be used to describe the NH2 group. Potential-optimized discrete variable representation basis is applied for the vibrational coordinates of the reagent NH3. The reaction probabilities for several initial vibrational states are presented in this paper. The role of the different vibrational excitations on the reactivity is thoroughly described.

MD/QM modeling of the modified gold nanoparticles and investigation of their sensing ability for selective detection of melamine

Full atomistic molecular dynamic (MD) simulations and quantum mechanics (QM) calculations were carried out to investigate the aggregation process of the pure and functionalized gold nanoparticles (AuNPs). On the basis of the obtained results, pure AuNPs are not stable in water, while 1‑(2‑mercaptoethyl)‑1,3,5‑triazinane‑2,4,6‑trione (MTT) functionalized gold nanoparticles (MTT-AuNPs) show a considerable stability in water. MTT groups change the surface properties of AuNPs and increase their resistance against aggregation. Moreover, MTT-AuNPs have an interesting ability for selective detection of melamine in the presence of different analytes, due to hydrogen bond formation of NH⋯N and NH⋯O. Moreover, electrostatic interactions are the driving force of the selective detection of melamine by MTT-AuNPs. A good agreement between the theoretical and experimental results confirms the ability of the theoretical methods for simulating the complex systems and designing new sensors based on the functionalized AuNPs.