Toward more accurate adiabatic connection approach for multireference wave functions

TL;DR

This paper explores removing the fixed-RDM approximation in multiconfigurational adiabatic connection methods to enhance the accuracy of dynamic correlation calculations in multireference wave functions.

Contribution

It investigates the impact of eliminating the fixed-RDM approximation in AC models, demonstrating potential improvements in accuracy for multireference wave function methods.

Findings

Lifting the fixed-RDM approximation improves AC accuracy.

Exact RDMs provide better reference integrands.

Second-order RDM expansions are effective in AC models.

Abstract

A multiconfigurational adiabatic connection (AC) formalism is an attractive approach to computing dynamic correlation within CASSCF and DMRG models. Practical realizations of AC have been based on two approximations: i) fixing one- and two-electron reduced density matrices (1- and 2-RDMs) at the zero-coupling constant limit and ii) extended random phase approximation (ERPA). This work investigates the the effect of removing the "fixed-RDM" approximation in AC. The analysis is carried out for two electronic Hamiltonian partitionings: the group product function- and the Dyall-Hamiltonians. Exact reference AC integrands are generated from the DMRG FCI solver. Two AC models are investigated, employing either exact 1- and 2-RDMs or their second-order expansions in the coupling constant in the ERPA equations. Calculations for model molecules indicate that lifting the fixed-RDM approximation is a viable way toward improving accuracy of the existing AC approximations.