Fully quantum non-adiabatic dynamics in electronic-nuclear coherent state basis

TL;DR

This paper introduces a novel quantum non-adiabatic dynamics method using a valence bond basis, enabling exact matrix element calculations and on-the-fly propagation of coherent states without approximations.

Contribution

It develops a new approach combining valence bond theory with coherent state dynamics for accurate, efficient quantum simulations of non-adiabatic processes.

Findings

Exact matrix elements for potential energy surfaces using valence bond theory

Analytical functions of nuclear coordinates for matrix elements

On-the-fly propagation of coherent states without approximations

Abstract

Direct dynamics methods using Gaussian wavepackets have to rely only on local properties, such as gradients and hessians at the center of the wavepacket, so as to be compatible with the usual quantum chemistry methods. Matrix elements of the potential energy surfaces between wavepackets therefore usually have to be approximated. It is shown, that if a modified form of valence bond theory is used instead of the usual MO-based theories, the matrix elements can be obtained exactly. This is so because the molecular Hamiltonian only contains the Coulomb potential, for which matrix elements between different basis functions (consisting of Gaussian nuclear and electronic orbitals) are all well-known. In valence bond theory the self-consistent field calculation can be avoided so that the matrix elements are analytical functions of the nuclear coordinates. A method for simulating non-adiabatic quantum dynamics is sketched, where coherent state trajectories are propagated "on the fly" on adiabatic potential energy surfaces without making approximations to the matrix elements responsible for the coupling between trajectories.