BALANCED COMPLETE ACTIVE SPACE CHOICES WITH THE MULTICONFIGURATIONAL SPIN TENSOR ELECTRON PROPAGATOR METHOD - THE VERTICAL IONIZATION-POTENTIALS OF NH2

With electron propagator methods, vertical ionization potentials (IPs) and electron affinities can be calculated directly. Our implementatio n, known as the multiconfigurational spin tensor electron propagator m ethod (MCSTEP), is specifically designed for open-shell and highly cor related initial states. The initial state that is usually used in MCST EP is a complete active space (CAS) MCSCF state. We have previously de monstrated that a small balanced CAS is the optimal choice for the MCS TEP initial state, in this article, we examine two ways of determining this balanced CAS for the MCSTEP initial state. With these choices, w e calculate the low-lying vertical MCSTEP IPs of NH, and compare them with experiment for the three lowest IPs. With (aug-)cc-pVTZ, (aug-)cc -pVQZ, and extrapolated complete basis set limits, the MCSTEP IPs for both CAS choices are in very good agreement with experiment. We also c alculate the next two vertical IPs where no accurate experimental esti mates are as yet available. The ''SCF-balanced'' CAS choice is advanta geous to use with MCSTEP because of simplicity, while the ''SDCI-balan ced'' CAS is appropriate for use with MCSTEP when the initial MCSCF ha s undesired properties. (C) 1998 John Wiley & Sons, Inc.