A SMALL OPTIMAL COMPLETE ACTIVE SPACE (CAS) FOR MULTICONFIGURATIONAL SPIN TENSOR ELECTRON PROPAGATOR METHOD (MCSTEP) IONIZATION-POTENTIALS - APPLICATION TO METHANE, ACETYLENE, ETHYLENE, AND ETHANE

The multiconfigurational spin tensor electron propagator method (MCSTE P) is specifically designed to provide accurate determination of the l ow-lying principal vertical ionization potentials (IP's) and electron affinities (EA's) of atoms and molecules. In this paper we modify and extend our definition of an ''optimal'' small complete active space (G AS) for MCSTEP calculations. We then use MCSTEP to determine the verti cal principal ionization potentials (IP's) below 19 eV of the small or ganic molecules methane, acetylene, ethylene, and ethane. The calculat ions are done using the standard Dunning-core-valence pVDZ and pVTZ ba sis sets. The results art: compared with experiments as well as with s ome other forefront theoretical methods. The MCSTEP IP's are in very g ood to excellent agreement with experimental IP's. (C) 1997 American I nstitute of Physics.