Hybrid quantum/semiclassical wave packet method for molecular dynamics: Application to photolysis of Ar...HCl

Calculation of the Transport and Relaxation Properties of the Ar···HCl van der Waals Complex Using a New Potential Energy Surface: Comparison of the Classical and Full Quantum Mechanical Kinetic Theory Results with Molecular Dynamics Simulations

The intermolecular potential energy surface (PES) of the Ar···HCl complex was calculated at the RCCSD(T)/aug-cc-pvQz-BF level of theory. The obtained potential was expanded in terms of Legendre polynomials and fitted to a mathematical model. The fitting results are highly correlated with the ab initio PES data with SD = 5.9 × 10^-3 cm^-1 and average absolute deviation (AAD) = 4.0 × 10^-6 cm^-1. The interaction second virial coefficients (B₁₂) in the temperature range of 190-480 K were calculated by considering classical and first quantum corrections and compared with the available experimental data. A reasonable agreement with the experimental and calculated B₁₂ was obtained. The PES was also used to obtain the rovibrational energy levels, and the spectroscopic rovibrational constants were obtained. It was found that the D₀ values differ ∼2.25 cm^-1 from the experimental values of the ground rovibrational state. Furthermore, the obtained potential was used to calculate the transport and relaxation properties using full quantum close-coupling (CC) formalism and the classical kinetic theory methods based on the Mason-Monchik approximation (MMA). It was found that the deviation between MMA and CC calculations is increased with increasing the temperature due to the higher influence of the rotational degrees of freedom on the transport properties. Also, the contribution of the inelastic (off-diagonal) transitions for diffusion coefficient is higher than the viscosity. Furthermore, the classical molecular dynamics simulations were performed using LJ(12,6) and Vashishta models, to calculate the interaction diffusion and viscosity coefficients, and compared with the results of the full quantum CC calculations. The obtained results confirm that the Vashishta model is better fitted to the ab initio potentials and is more accurate than LJ(12,6) in calculation of the diffusion coefficients.

My Trajectory in Molecular Reaction Dynamics and Spectroscopy

This is the story of a career in theoretical chemistry during a time of dramatic changes in the field due to phenomenal growth in the availability of computational power. It is likewise the story of the highly gifted graduate students and postdoctoral fellows that I was fortunate to mentor throughout my career. It includes reminiscences of the great mentors that I had and of the exciting collaborations with both experimentalists and theorists on which I built much of my research. This is an account of the developments of exciting scientific disciplines in which I was involved: vibrational spectroscopy, molecular reaction mechanisms and dynamics, e.g., in atmospheric chemistry, and the prediction of new, exotic molecules, in particular noble gas molecules. From my very first project to my current work, my career in science has brought me the excitement and fascination of research. What a wonderful pursuit!

Wave packet study of the UV photodissociation of the Ar2HBr complex

The photodissociation dynamics of the Ar2HBr van der Waals molecule is studied using the multiconfiguration time-dependent Hartree method. Standard Jacobian coordinates are used to describe the molecule. Two four-dimensional calculations are carried out where the rotation of the Ar2 molecule and, in addition, either the vibration of Ar2-Br or that of Ar2 are frozen. The time-evolution of the probability density in the different modes and the calculation of the dissociative flux show that the dissociating hydrogen atom preferentially moves out of the plane defined by Ar2 and Br. A comprehensive study of the cage effect in the process is presented.

A canonical averaging in the second-order quantized Hamilton dynamics

Quantized Hamilton dynamics (QHD) is a simple and elegant extension of classical Hamilton dynamics that accurately includes zero-point energy, tunneling, dephasing, and other quantum effects. Formulated as a hierarchy of approximations to exact quantum dynamics in the Heisenberg formulation, QHD has been used to study evolution of observables subject to a single initial condition. In present, we develop a practical solution for generating canonical ensembles in the second-order QHD for position and momentum operators, which can be mapped onto classical phase space in doubled dimensionality and which in certain limits is equivalent to thawed Gaussian. We define a thermal distribution in the space of the QHD-2 variables and show that the standard beta=1/kT relationship becomes beta'=2/kT in the high temperature limit due to an overcounting of states in the extended phase space, and a more complicated function at low temperatures. The QHD thermal distribution is used to compute total energy, kinetic energy, heat capacity, and other canonical averages for a series of quartic potentials, showing good agreement with the quantum results.