Automatic code generation enables nuclear gradient computations for fully internally contracted multireference theory

TL;DR

This paper presents an automated code generator for analytical nuclear gradients in CASPT2, enabling efficient and accurate computations of molecular properties for complex systems.

Contribution

It introduces a novel automated code generation approach for derivatives in fully internally contracted multireference theories, improving computational efficiency.

Findings

Successfully computed ionization potentials of porphin

Demonstrated capability for vertical and adiabatic calculations

Enhanced accuracy and automation in multireference gradient computations

Abstract

Analytical nuclear gradients for fully internally contracted complete active space second-order perturbation theory (CASPT2) are reported. This implementation has been realized by an automated code generator that can handle spin-free formulas for the CASPT2 energy and its derivatives with respect to variations of molecular orbitals and reference coefficients. The underlying complete active space self-consistent field and the so-called Z-vector equations are solved using density fitting. The implementation has been applied to the vertical and adiabatic ionization potentials of the porphin molecule to illustrate its capability.