The determination of electron affinities of the open shell systems C and CH₂ using the multiconfigurational spin tensor electron propagator method

The multiconfigurational spin tensor electron propagator method (MCSTEP) is a Green's function approach for accurately predicting and analyzing ionization potentials (IPs) and electron affinities (EAs). Unlike more traditional Green's function approaches, MCSTEP is applicable to highly correlated and open shell systems as well as to closed shell systems with small correlation effects. We apply MCSTEP for the determination of EAs for C and CH₂. This is the first time that MCSTEP has been used to determine the EAs for systems which have both open shell neutral and anionic ground states. Our best MCSTEP results for the EA of C and the adiabatic EA of CH₂ are 1.2904 and 0.6356 eV, respectively, compared to 1.2607 and 0.6306 eV, respectively, obtained with large scale multireference configuration interaction (MRCI) using the same basis sets. Experimental values are 1.268 eV for C and 0.628±0.031 eV for CH₂. We also show that accurate EAs for these systems can be obtained both with MRCI and especially with MCSTEP using moderately sized basis sets composed of a good, standard core-valence basis set supplemented with some additional diffuse functions.