Slow dynamics of the high density Gaussian core model

We numerically study crystal nucleation and glassy slow dynamics of the one-component Gaussian core model (GCM) at high densities. The nucleation rate at a fixed supercooling is found to decrease as the density increases. At very high densities, the nucleation is not observed at all in the time window accessed by long molecular dynamics (MD) simulation. Concomitantly, the system exhibits typical slow dynamics of the supercooled fluids near the glass transition point. We compare the simulation results of the supercooled GCM with the predictions of mode-coupling theory (MCT) and find that the agreement between them is better than any other model glassformers studied numerically in the past. Furthermore, we find that a violation of the Stokes-Einstein relation is weaker and the non-Gaussian parameter is smaller than canonical glassformers. Analysis of the probability distribution of the particle displacement clearly reveals that the hopping effect is strongly suppressed in the high density GCM. We conclude from these observations that the GCM is more amenable to the mean-field picture of the glass transition than other models. This is attributed to the long-ranged nature of the interaction potential of the GCM in the high density regime. Finally, the intermediate scattering function at small wavevectors is found to decay much faster than its self part, indicating that dynamics of the large-scale density fluctuations decouples with the shorter-ranged caging motion.     

Reference interaction site model and molecular dynamics study of structure and thermodynamics of methanol

Thermodynamic and structural properties of various models of liquid methanol are investigated in the framework provided by the reference interaction site model (RISM) theory of molecular fluids. The theoretical predictions are systematically compared with molecular dynamics simulations both at ambient conditions and along a few supercritical isotherms. RISM results for the liquid-vapor phase separation are also obtained and assessed against available Gibbs ensemble Monte Carlo data. At ambient conditions, the theoretical correlations weakly depend on the specific details of the molecular models and reproduce the simulation results with different degrees of accuracy, depending on the pair of interaction sites considered. The position and the strength of the hydrogen bond are quite satisfactorily predicted. RISM results for the internal energy are almost quantitative whereas the pressure is generally overestimated. As for the liquid-vapor phase coexistence, RISM predictions for the vapor branch and for the critical temperature are quite accurate; on the other side, the liquid branch densities, and consequently the critical density, are underestimated. We discuss our results in terms of intrinsic limitations, and suitable improvements, of the RISM approach in describing the physical properties of polar fluids, and in the perspective of a more general investigation of mixtures of methanol with nonpolar fluids of specific interest in the physics of associating fluids.     

Two-Gaussian excitations model for the glass transition

We develop a modified "two-state" model with Gaussian widths for the site energies of both ground and excited states, consistent with expectations for a disordered system. The thermodynamic properties of the system are analyzed in configuration space and found to bridge the gap between simple two-state models ("logarithmic" model in configuration space) and the random energy model ("Gaussian" model in configuration space). The Kauzmann singularity given by the random energy model remains for very fragile liquids but is suppressed or eliminated for stronger liquids. The sharp form of constant-volume heat capacity found by recent simulations for binary mixed Lennard-Jones and soft-sphere systems is reproduced by the model, as is the excess entropy and heat capacity of a variety of laboratory systems, strong and fragile. The ideal glass in all cases has a narrow Gaussian, almost invariant among molecular and atomic glassformers, while the excited-state Gaussian depends on the system and its width plays a role in the thermodynamic fragility. The model predicts the possibility of first-order phase transitions for fragile liquids. The analysis of laboratory data for toluene and o-terphenyl indicates that fragile liquids resolve the Kauzmann paradox by a first-order transition from supercooled liquid to ideal-glass state at a temperature between T(g) and Kauzmann temperature extrapolated from experimental data. We stress the importance of the temperature dependence of the energy landscape, predicted by the fluctuation-dissipation theorem, in analyzing the liquid thermodynamics.     

Molecular thermodynamics model for random copolymer solutions

Similar to the work done by Kambour et al. and ten Brinke et al., an effective interchange energy parameter ɛ^eff is introduced into the calculation of binodal curves for random copolymer systems according to the Revised Freed (R-F) model previously established by Hu et al. for homopolymer system. ɛ^eff is a function of chain composition and various pairs of energy parameters. For contrast, the Flory-Huggins (F-H) model has also been extended to study random copolymer system in the similar way. In order to check the validity of this kind of approach, the calculated results have been compared with the corresponding MC simulation data. It is shown that the results obtained by R-F model are quite consistent with MC simulation data while that by F-H model shows large deviation due to the excess mean-field approximation employed.     

Cellular model for zeolite gas adsorption thermodynamics

Zeolite structures allow one to use a cellular adsorption model of liquid type, which can be used to describe adsorption for a gas mixture in which adsorption forms differing in orientation can arise for molecules of a single kind, which may be accompanied by dissociative or multicentered adsorption, in which an adsorbate molecule occupies several adjacent centers. General adsorption-isotherm equations have been derived together with expressions for the major integral and differential thermodynamic functions (free and internal energies, entropy, and specific heat) in gas-mixture adsorption in the cellular model. The adsorption isotherms in it give a one-to-one correspondence between the gas composition and that in the adsorption layer at all temperatures and degrees of filling, so they can be used for regular-lattice adsorbents, but only outside the critical region.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 584–589, September–October, 1988.     

Isomerization dynamics and thermodynamics of ionic argon clusters

The dynamics and thermodynamics of small Ar(n) (+) clusters, n=3, 6, and 9, are investigated using molecular dynamics (MD) and exchange Monte Carlo (MC) simulations. A diatomic-in-molecule Hamiltonian provides an accurate model for the electronic ground state potential energy surface. The microcanonical caloric curves calculated from MD and MC methods are shown to agree with each other, provided that the rigorous conservation of angular momentum is accounted for in the phase space density of the MC simulations. The previously proposed projective partition of the kinetic energy is used to assist MD simulations in interpreting the cluster dynamics in terms of inertial, internal, and external modes. The thermal behavior is correlated with the nature of the charged core in the cluster by computing a dedicated charge localization order parameter. We also perform systematic quenches to establish a connection with the various isomers. We find that the Ar(3) (+) cluster is very stable in its linear ground state geometry up to about 300 K, and then isomerizes to a T-shaped isomer in which a quasineutral atom lies around a charged dimer. In Ar(6) (+) and Ar(9) (+), the covalent trimer core is solvated by neutral atoms, and the weakly bound solvent shell melts at much lower energies, occasionally leading to a tetramer or pentamer core with weakly charged extremities. At high energies the core itself becomes metastable and the cluster transforms into Ar(2) (+) solvated by a fluid of neutral argon atoms.     

Electron correlation via frozen Gaussian dynamics

We investigate the accuracy and efficiency of the semiclassical frozen Gaussian method in describing electron dynamics in real time. Model systems of two soft-Coulomb-interacting electrons are used to study correlated dynamics under non-perturbative electric fields, as well as the excitation spectrum. The results show that a recently proposed method that combines exact-exchange with semiclassical correlation to propagate the one-body density-matrix holds promise for electron dynamics in many situations that either wavefunction or density-functional methods have difficulty describing. The results also however point out challenges in such a method that need to be addressed before it can become widely applicable.     

Constrained dynamics of localized excitations causes a non-equilibrium phase transition in an atomistic model of glass formers

Recent progress has demonstrated that trajectory space for both kinetically constrained lattice models and atomistic models can be partitioned into a liquid-like and an inactive basin with a non-equilibrium phase transition separating these behaviors. Recent work has also established that excitations in atomistic models have statistics and dynamics like those in a specific class of kinetically constrained models. But it has not been known whether the non-equilibrium phase transitions occurring in the two classes of models have similar origins. Here, we show that the origin is indeed similar. In particular, we show that the number of excitations identified in an atomistic model serves as the order parameter for the inactive-active phase transition for that model. In this way, we show that the mechanism by which excitations are correlated in an atomistic model - by dynamical facilitation - is the mechanism from which the active-inactive phase transition emerges. We study properties of the inactive phase and show that it is amorphous lacking long-range order. We also discuss the choice of dynamical order parameters.     

Theoretical study of excitations in furan: spectra and molecular dynamics

The excitation spectra and molecular dynamics of furan associated with its low-lying excited singlet states 1A2(3s), 1B2(V), 1A1(V'), and 1B1(3p) are investigated using an ab initio quantum-dynamical approach. The ab initio results of our previous work [J. Chem. Phys. 119, 737 (2003)] on the potential energy surfaces (PES) of these states indicate that they are vibronically coupled with each other and subject to conical intersections. This should give rise to complex nonadiabatic nuclear dynamics. In the present work the dynamical problem is treated using adequate vibronic coupling models accounting for up to four coupled PES and thirteen vibrational degrees of freedom. The calculations were performed using the multiconfiguration time-dependent Hartree method for wave-packet propagation. It is found that in the low-energy region the nuclear dynamics of furan is governed mainly by vibronic coupling of the 1A2(3s) and 1B2(V) states, involving also the 1A1(V') state. These interactions are responsible for the ultrafast internal conversion from the 1B2(V) state, characterized by a transfer of the electronic population to the 1A2(3s) state on a time scale of approximately 25 fs. The calculated photoabsorption spectrum of furan is in good qualitative agreement with experimental data. Some assignments of the measured spectrum are proposed.     

Structure,dynamics and thermodynamics of small particles and inorganic clusters

This invited review attempts to draw together recent advances in the structural characterisation of clusters and our theoretical understanding of dynamics, especially coexistence phenomena. It is now possible to characterise the potential energy surface of a small cluster in great detail, both in terms of local minima and transition states. A selection of results is collected includingab initio calculations on main group ligated clusters and a wide variety of systems bound by model analytic potentials. Useful comparisons may be made between the rearrangement mechanisms supported by the various potential energy surfaces. Furthermore, knowledge of transition states enables us to explain the results of dynamical simulations in great detail, and make comparisons with thermodynamic models. For larger systems, however, the number of stationary points is daunting, yet progress is still possible in terms of the underlying potential energy surface using the harmonic superposition approximation.     

Extension of EWMO model to particle-hole excitations

Examination of the random phase approximation in relation to the energy weighted maximum overlap model suggests a way of accounting for singlet–triplet splittings in particle–hole spectra. Calculations for the first row hydride molecules are used to illustrate applications.     

Particlelike excitations in the polypeptide chain model

The α-helical protein molecule is modeled by a one-dimensional crystal chain with three peptide groups in the lattice cell and with the nearest and third neighbor interactions. Three types of solitonlike solutions describing excitation motion along the model have been obtained. The first one is three modulated waves which are in phase. The other two are modulated waves with phase shifts equal to ?π and \documentclass{article}\pagestyle{empty}\begin{document}$ {\textstyle{4 \over 3}} $\end{document}π, respectively.     

The Gaussian and augmented-plane-wave density functional method for ab initio molecular dynamics simulations

A new algorithm for density-functional-theory-based ab initio molecular dynamics simulations is presented. The Kohn–Sham orbitals are expanded in Gaussian-type functions and an augmented-plane-wave-type approach is used to represent the electronic density. This extends previous work of ours where the density was expanded only in plane waves. We describe the total density in a smooth extended part which we represent in plane waves as in our previous work and parts localised close to the nuclei which are expanded in Gaussians. Using this representation of the charge we show how the localised and extended part can be treated separately, achieving a computational cost for the calculation of the Kohn–Sham matrix that scales with the system size N as O(NlogN). Furthermore, we are able to reduce drastically the size of the plane-wave basis. In addition, we introduce a multiple-cutoff method that improves considerably the performance of this approach. Finally, we demonstrate with a series of numerical examples the accuracy and efficiency of the new algorithm, both for electronic structure calculations and for ab initio molecular dynamics simulations. Received: 15 December 1998 /Accepted: 18 February 1999 /Published online: 14 July 1999     

Thermal Gaussian molecular dynamics for quantum dynamics simulations of many-body systems: application to liquid para-hydrogen

A new method, here called thermal Gaussian molecular dynamics (TGMD), for simulating the dynamics of quantum many-body systems has recently been introduced [I. Georgescu and V. A. Mandelshtam, Phys. Rev. B 82, 094305 (2010)]. As in the centroid molecular dynamics (CMD), in TGMD the N-body quantum system is mapped to an N-body classical system. The associated both effective Hamiltonian and effective force are computed within the variational Gaussian wave-packet approximation. The TGMD is exact for the high-temperature limit, accurate for short times, and preserves the quantum canonical distribution. For a harmonic potential and any form of operator A?, it provides exact time correlation functions C(AB)(t) at least for the case of B, a linear combination of the position, x, and momentum, p, operators. While conceptually similar to CMD and other quantum molecular dynamics approaches, the great advantage of TGMD is its computational efficiency. We introduce the many-body implementation and demonstrate it on the benchmark problem of calculating the velocity time auto-correlation function for liquid para-hydrogen, using a system of up to N = 2592 particles.     

First-principles study of lattice dynamics and thermodynamics of TiO2 polymorphs

The structural, phonon, and thermodynamic properties of six TiO(2) polymorphs, i.e., rutile, anatase, columbite, baddeleyite, orthorhombic I, and cotunnite, have been systematically investigated by density functional theory. The predicted volumes, bulk modulus, and Debye temperature are in good agreement with experiments. The phonon dispersions of the TiO(2) polymorphs were studied by the supercell approach, whereas the long-range dipole-dipole interactions were calculated by linear response theory to reproduce the LO-TO splitting, making accurate prediction of phonon frequencies for the polar material TiO(2). The calculated phonon dispersions show that all TiO(2) polymorphs are dynamically stable at ambient pressure, indicating the high-pressure phases might be quenched to ambient conditions as ultrahard materials. Furthermore, the finite temperature thermodynamic properties of TiO(2) polymorphs were predicted accurately from the obtained phonon density of states, which is critical in the future study of the pressure-temperature phase diagram of TiO(2). The calculated Gibbs energies reveal that rutile is more stable than anatase at ambient pressure. We derived the Gibbs energy and heat capacity functions for all TiO(2) polymorphs for use in thermodynamic modeling of phase equilibria.     

Gaussian induced dipole polarization model

A new induced dipole polarization model based on interacting Gaussian charge densities is presented. In contrast to the original induced point dipole model, the Gaussian polarization model is capable of finite interactions at short distances. Aspects of convergence related to the Gaussian model will be explored. The Gaussian polarization model is compared with the damped Thole-induced dipole model and the point dipole model. It will be shown that the Gaussian polarization model performs slightly better than the Thole model in terms of fitting to molecular polarizability tensors. An advantage of the model based on Gaussian charge distribution is that it can be easily generalized to other multipole moments and provide effective damping for both permanent electrostatic and polarization models. Finally, a method of parameterizing polarizabilities is presented. This method is based on probing a molecule with point charges and fitting polarizabilities to electrostatic potential. In contrast to the generic atom type polarizabilities fit to molecular polarizability tensors, probed polarizabilities are significantly more accurate in terms of reproducing molecular polarizability tensors and electrostatic potential, while retaining conformational transferability.     

The nature and dynamics of nonlinear excitations in conducting polymers. Polyaniline

The advantages of using the multifrequency (3-cm, 8-mm, and 2-mm) EPR method in combination with the modified method of spin label and spin probes and the methods of steady-state microwave saturation and microwave saturation transfer of spin packets in studies of the spin and electrodynamic properties of polyaniline are outlined. The methods of determining the composition of localized and delocalized paramagnetic centers and calculating their main parameters from total ESR spectra are described. Original results of investigations of polyaniline modified with sulfuric, hydrochloric, camphorsulfonic, and 2-acrylamido-2-methyl-l-propanesulfonic acids are reported. The dependences of the nature, electronic relaxation, dynamics of paramagnetic centers, and the charge-transfer mechanism on the method of synthesis, the structure of the acid molecule, and the polyaniline oxidation level are shown. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 205–230, February, 2000.