Analytic Gradients for Complete Active Space Pair-Density Functional Theory

TL;DR

This paper develops analytical gradient routines for MC-PDFT with a state-specific CASSCF reference, enabling efficient geometry optimization with results comparable to CASPT2 but at lower computational cost.

Contribution

It introduces a variational Lagrangian approach for analytical gradients in MC-PDFT with a CASSCF reference, improving efficiency in molecular geometry calculations.

Findings

MC-PDFT accurately locates equilibrium geometries

Results are similar to CASPT2 but with reduced computational cost

Method enables efficient geometry optimizations for organic molecules

Abstract

Analytic gradient routines are a desirable feature for quantum mechanical methods, allowing for efficient determination of equilibrium and transition state structures and several other molecular properties. In this work, we present analytical gradients for multiconfiguration pair-density functional theory (MC-PDFT) when used with a state-specific complete active space self-consistent field reference wave function. Our approach constructs a Lagrangian that is variational in all wave function parameters. We find that MC-PDFT locates equilibrium geometries for several small- to medium-sized organic molecules that are similar to those located by complete active space second-order perturbation theory but that are obtained with decreased computational cost.