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 (MCSTEP) is specifically designed to provide accurate determination of the low-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 (CAS) for MCSTEP calculations. We then use MCSTEP to determine the vertical principal ionization potentials (IP’s) below 19 eV of the small organic molecules methane, acetylene, ethylene, and ethane. The calculations are done using the standard Dunning-core-valence pVDZ and pVTZ basis sets. The results are compared with experiments as well as with some other forefront theoretical methods. The MCSTEP IP’s are in very good to excellent agreement with experimental IP’s.