Efficient multireference perturbation theory without high-order reduced density matrices

TL;DR

This paper introduces a stochastic method for strongly contracted NEVPT that efficiently computes perturbation energies using only low-order reduced density matrices, applicable to various wave functions.

Contribution

The authors develop a stochastic SC-NEVPT approach that avoids high-order RDMs, enabling efficient and accurate multireference perturbation calculations for large systems.

Findings

Method achieves accurate energies for small test systems.

Energy is insensitive to reference wave function quality.

Scales favorably with system size and number of virtual orbitals.

Abstract

We present a stochastic approach to perform strongly contracted n-electron valence state perturbation theory (SC-NEVPT), which only requires one- and two-body reduced density matrices, without introducing approximations. We use this method to perform SC-NEVPT2 for CASSCF wave functions obtained from selected configuration interaction, although the approach is applicable to a larger class of wave functions, including those from variational Monte Carlo (VMC). The accuracy of this approach is demonstrated for small test systems, and the scaling is investigated with the number of virtual orbitals and the molecule size. We also find the SC-NEVPT2 energy to be relatively insensitive to the quality of the reference wave function. Finally, the method is applied to the Fe(II)-Porphyrin system with a (32e, 29o) active space, and to the isomerization of Cu2O2 in a (28e, 32o) active space.