EOMCC, MRPT, and TDDFT Studies of Charge Transfer Processes in Mixed-Valence Compounds: Application to the Spiro Molecule

The proper description of electron transfer (ET) processes in mixed-valence compounds poses a significant challenge for commonly used theoretical approaches. In this paper we analyze the 1(2)A(2) and 2(2)A(2) potential energy surfaces of the Spiro cation (5,5'(4H,4H')-spirobitcyclopenta[c]pyrrole]2,2',6,6'-tetrahydro cation) which is a frequently used model to study ET processes We compare and contrast the results obtained with three different methods multireference perturbation theory, equation-of-motion coupled cluster theory, time-dependent density functional theory. We demonstrate that the proper inclusion of dynamical correlation effects plays a crucial role in the description of an avoided crossing between potential energy surfaces. We also find that proper balancing of the ground- and excited-state correlation effects is especially challenging in the vicinity of the 1(2)A(2) and 2(2)A(2) avoided crossing region.