Active Space Requirements for State Specific MRPT and MRAQCC Methods

Author Institution: Departments of Chemistry and The Laser Spectroscopy Facility, The Ohio Stale UniversityThe recent formulation of Stale Specific Multireference Perturbation Theory and the Multireference Averaged Quadratic Coupled Cluster methods allows for treatment of dynamic and non-dynamic electron correlation which is approximately size consistent. The focus of the present work is to explore the active space requirements for MRACPF, MRAQCC, MRCISD, and MRPT methods. Spectroscopic constants, excitation energies, and electron affinities have been computed with a wide range of active spaces for the chromophore involving the $C_{2} X \ {^{1}}\Sigma_{g}^{+} , C_{2} a \ {^{3}}\Pi_{u}, C_{2} -X \ {^{2}}\Sigma_{g}^{+},\ C_{2} - A \ {^{2}}\Pi_{u},$ and the $C_{2}^{-} B \ {^{2}} \Sigma_{u}^{+}$ states