A transformed framework for dynamic correlation in multireference problems

TL;DR

This paper introduces a new framework that simplifies multireference dynamic correlation calculations by transforming them into single-reference problems using canonical transformations, avoiding common instabilities.

Contribution

It presents a novel canonically transformed approach that makes multireference correlation theories resemble single-reference methods, simplifying calculations and improving stability.

Findings

Formulated Bogoliubov transformed second-order Møller-Plesset theory

Demonstrated effectiveness in bond dissociation cases

Avoids high-order density matrix complications

Abstract

We describe how multirefence dynamic correlation theories can be naturally obtained as single-reference correlation theories in a canonically transformed frame. Such canonically transformed correlation theories are very simple and involve identical expressions to their single-reference counterparts. The corresponding excitations involve quasiparticles rather than the bare particles of the system. High-order density matrices (or their approximations) and the numerical metric instabilities common to multireference correlation theories do not appear. As an example, we formulate the Bogoliubov canonically transformed version of second-order M{\o}ller-Plesset perturbation theory and demonstrate its performance in hydrogen, water, nitrogen, and BeH$_2$ bond dissociation.