MCSTEP‐ A Multiconfigurational‐based Spin Adapted Electron Propagator Approach for Ionization Potentials and Electron Affinities

Abstract

AbstractWith electron propagator (also known assingle particle Green's function) techniques, electron removal and attachment energies are calculated directly. This avoids the sometimes inaccurate process of separately determining the total electronic energies of the neutral and ionic state and subtracting one large number from another to obtain a relatively small value, that is, the ionization potential (IP) or electron affinity (EA) of a molecule. Traditionally, these electron propagator methods used a single determinant wave function as the ‘zero order’ initial state, which was improved with Møller–Plesset perturbation theory. Although these usual perturbative electron propagator methods have been very successful, they are limited in applicability. Usual perturbative approaches usually cannot handle reliably (or at all) systems with initial states that are open shell and/or highly correlated (nondynamical correlation) for either IPs or EAs.We specifically designed the multiconfigurational spin tensor electron propagator method (MCSTEP) and its predecessor the multiconfigurational electron propagator (MCEP) method to provide accurate IPs and EAs for systems that cannot be accurately handled by usual perturbational approaches to single particle Green's function methods, namely, when the initial state is open shell and/or has nondynamical correlation that must be accounted for. In addition, of course, the goal is to also be able to provide accurate IPs and EAs for systems with closed shell initial states without nondynamical correlation, that is, those systems that could be handled as well by usual perturbational electron propagator methods.In this article, I will first review the theory behind the multiconfigurational spin tensor electron propagator method. Since the introduction of MCSTEP over 15 years ago, several accurate MCSTEP atomic and molecular IPs and EAs have been determined. I will summarize several of the more significant calculations to date.An electron propagator method using a multiconfigurational second‐order perturbation theory wave function as the initial state in the fermion operator block (block 1) in the MCSTEP matrix equations was initially developed by Heryadi and Yeager. In the other blocks, an MCSCF wave function is the initial state. This new method is called EPCASPT2 and should be viewed as an extension of MCSTEP. In this article, we will review the theory behind EPCASPT2 and some of the recent calculations done using a CASPT2 wave function as the initial state in the electron propagator. We compare our results with the results of the calculations using multiconfigurational spin tensor electron propagator, full configuration interaction, and for the molecules, the multireference configuration interaction method with the same geometries and basis sets.