Systematic design of active spaces for multi-reference calculations of singlet-triplet gaps of organic diradicals, with benchmarks against doubly electron-attached coupled-cluster data

Samuel J Stoneburner; Jun Shen; Adeayo O Ajala; Piotr Piecuch; Donald G Truhlar; Laura Gagliardi

doi:10.1063/1.4998256

Systematic design of active spaces for multi-reference calculations of singlet-triplet gaps of organic diradicals, with benchmarks against doubly electron-attached coupled-cluster data

J Chem Phys. 2017 Oct 28;147(16):164120. doi: 10.1063/1.4998256.

Authors

Samuel J Stoneburner¹, Jun Shen², Adeayo O Ajala², Piotr Piecuch², Donald G Truhlar¹, Laura Gagliardi¹

Affiliations

¹ Department of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455-0431, USA.
² Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.

PMID: 29096487
DOI: 10.1063/1.4998256

Abstract

Singlet-triplet gaps in diradical organic π-systems are of interest in many applications. In this study, we calculate them in a series of molecules, including cyclobutadiene and its derivatives and cyclopentadienyl cation, by using correlated participating orbitals within the complete active space (CAS) and restricted active space (RAS) self-consistent field frameworks, followed by second-order perturbation theory (CASPT2 and RASPT2). These calculations are evaluated by comparison with the results of doubly electron-attached (DEA) equation-of-motion (EOM) coupled-cluster (CC) calculations with up to 4-particle-2-hole (4p-2h) excitations. We find active spaces that can accurately reproduce the DEA-EOMCC(4p-2h) data while being small enough to be applicable to larger organic diradicals.