Modification of optimal complete active space choices for the multiconfigurational spin-tensor electron propagator method for ionization potentials
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abstract
The multiconfiguratiorml spin tensor electron propagator (MCSTEP) method was developed as an implementation of electron propagator/single particle Green's function methods. MCSTEP was specifically designed for open-shell and highly correlated (nondynamically correlated) initial states. Ionization or electron attachment is always from a state of pure spin symmetry to a state of pure spin symmetry even it' the initial state is open shell. MCSTEP can he used as well for molecules with initial states that can be accurately described by a single determinant-based theory. The initial state that is used in MCSTEP is typically a small complete active space (CAS) multicunfigumtional self-consistent field (MCSCF) state. We previously examined different small CAS choices for MCSTEP initial states and have developed a generally workable scheme. This article further examines some different ways to choose the CAS for MCSTEP. With several logical CAS choices, we have calculated the low-lying vertical MCSTEP ionization potentials (IPs) of C2, N2, linear U2O, O2, CH2, and NH2. comparing them with large multireference configuration interaction (MRCI) calculations. We conclude that generally a small modification and extension of our previous schemes for choosing the MCSTEP CAS gives IPs that most effectively mimics the results of large scale MRCI IPs in general. 2006 Wiley Periodicals, Inc.
