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.
Bibliographical noteFunding Information:
The authors thank Molly Andersen for helpful assistance. This work was supported in part (S.J.S., D.G.T., and L.G.) by the NSF Grant No. CHE-1464536 for the multi-reference calculations (CASSCF, CASPT2, and RASPT2) and in part (P.P., J.S., and A.O.A.) by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Award No. DE-FG02-01ER15228 for the coupled-cluster calculations.
© 2017 Author(s).