Spin-flip pair-density functional theory: A practical approach to treat static and dynamical correlations in large molecules

TL;DR

This paper introduces a practical method combining spin-flip and on-top pair-density functional theories to accurately treat static and dynamical correlations in large molecules, demonstrated on various radical and polyacene systems.

Contribution

It develops a new approach integrating spin-flip and on-top pair-density functionals with external relaxations for large multi-configurational molecules.

Findings

Accurate energy gaps for diradicals and triradicals.

Effective treatment of large polyacene chains.

Demonstrated improved accuracy over previous methods.

Abstract

We present a practical approach to treat static and dynamical correlation accurately in large multi-configurational systems. The static correlation is accounted for using the spin-flip approach which is well known for capturing static correlation accurately at low-computational expense. Unlike previous approaches to add dynamical correlation to spin-flip models which use perturbation theory or coupled-cluster theory, we explore the ability to use the on-top pair-density functional theory approaches recently developed by Gagliardi and co-workers (JCTC, 10, 3669, 2014). External relaxations are carried out in the spin-flip calculations though a restricted active space framework for which a truncation scheme for the orbitals used in the external excitation is presented. The performance of the approach is demonstrated by computing energy gaps between ground and excited states for diradicals, triradicals and linear polyacene chains ranging from naphthalene to dodecacene. Accurate results are obtained using the new approach for these challenging open-shell molecular systems.