Approaches towards the full CI limit for singlet biradicals

Devin Matthews, University of Texas

Biradical systems present a challenge to traditional single-reference methods in quantum chemistry. Many methods have been proposed to study these important and computationally complicated systems, such as complete-active-space SCF and perturbation theory, multireference (and especially internally-contracted) configuration interaction (CI), and Fock-space multireference coupled-cluster. In this work traditional single-reference methods are compared against another type of multireference method, equation-of-motion double ionization potential coupled-cluster (EOMDIP-CC). Ozone, C2, nitrene (imidogen), and methylnitrene are chosen as test systems to sample a variety of molecular symmetries, number of valence electrons, and orbital configurations. A wide range of levels of excitation in the CC truncation are examined where possible in order to examine the approach of both methods to the exact Full CI limit. While it is determined that EOMDIP-CC can improve on single-reference methods in some cases, there are several limitations that must be addressed. Additionally, a new approximate triple-excitation method is proposed which has desirable similarities to CCSD(T) and CCSD(T)-a and which is easily incorporated into subsequent EOMDIP calculations. Further work will attempt to address some of the current shortcomings of EOMDIP-CC.

Abstract Author(s): Devin A. Matthews, John F. Stanton