A path integral Monte Carlo method for obtaining thermodynamic properties of nonadiabatic systems

Abstract

The calculation of thermochemical properties is an important goal of quantum chemistry. Calculation techniques are well established for stable molecules. They are used routinely to calculate Gibbs energy (G) for stable isomers, Gibbs energy difference (ΔG) for reactions and also to obtain activation energies in the context of transition state theory. Practical calculations use harmonic oscillator (H.O.), Rigid Rotor (RR) and ideal gas approximations to obtain thermodynamic contributions. This approach works well for many systems, but breaks down for systems with multiple low-lying electronic states. Examples of such systems are found among radicals, systems containing transition metal atoms, and open-shell states when spin-orbit coupling is considered. Systems of this type are better described by vibronic models acting through a small manifold of electronic states. In this work we describe a path integral Monte Carlo (PIMC) approach that, for vibronic models, will obtain the partition function (Z) and thermodynamic properties as a function of temperature. Investigation of model systems demonstrates that the partition function and internal energy can be obtained in an efficient manner.