Approximate MCSCF optimization for multiconfigurational spin‐tensor electron propagator method (MCSTEP): The vertical ionization potentials of CO, HCN, HNC, H₂CO, and O₃

Abstract

ABSTRACT:The multiconfigurational spin tensor electron propagator method (MCSTEP) was developed as an implementation of electron propagator/single particle Green's function methods. MCSTEP was specifically designed for open shell and highly correlated (nondynamically correlated) initial states. The initial state used in MCSTEP is typically a small complete active space (CAS) with multiconfigurational self-consistent field (MCSCF) state. In some cases, because of our use of a small CAS in MCSTEP, the Lagrangian eigenvalues of the MCSCF reference state are in an undesired order (u). The desired order (d) can usually be obtained by excluding one or more orbital rotations in MCSCF optimization between the doubly occupied and partially occupied orbitals. We systematically examine several cases where the undesired order occurs for the low-lying vertical MCSTEP ionization potentials (IPs) of the molecules CO, HCN, HNC, H 2 CO, and O 3 with our recently established CAS choices for MCSCF/MCSTEP. By excluding one or more orbital rotations between the partially and doubly occupied orbitals, an approximate MCSCF reference state with the same CAS choice is obtained for use in standard MCSTEP calculations that, in general, gives more reliable vertical MCSTEP IPs.

Obtaining positions and widths of scattering resonances from a complex multiconfigurational self‐consistent field state using the M1 method

“…Current work in our laboratory includes combining complex‐scaled electron‐nuclear attraction integral evaluation subroutine to our CMCSCF codes with application of the M 1 method to study shape, Auger and Feshbach resonances of molecules of chemical and physical interest. In addition, we are developing ways to determine the resonance positions and widths accurately using CMCSCF‐based electron propagator method (CMCSTEP) that utilizes the full M matrix, since MCSTEP does not use single determinant‐based perturbation theory and has been shown to give highly reliable IPs and EAs for atoms and molecules including open shell and highly correlated systems 30–32, 50–56.…”

“…Multiconfigurational spin tensor electron propagator (MCSTEP) is a very powerful tool for calculating accurate ionization potentials (IPs) and electron affinities (EAs) including open shell atoms and molecules that have nondynamical correlation in their initial (or reference) states 30–32, 50–56. It involves solving for IPs/EAs (ω f ) from the following eigenvalue equation 30: $\underline {\underline M}$ has five nonzero blocks and each one involves a combination of tensor operators.…”

Recent Advances in the Study of Negative‐Ion Resonances Using Multiconfigurational Propagator and a Complex Absorbing Potential

“…We showed earlier that the operator manifold $${\{{a}^{i},{a}_{i}\}}$$ for a multiconfigurational EP defined in terms of a complex scaled Hamiltonian can reproduce the resonance positions and widths for simple (shape) resonances very well [80,175,176] . Those were motivated by the work of Yeager and co‐workers on multiconfigurational electron propagator for the determination of ionization potential (IP) and electron affinity (EA) of highly correlated systems [294–305] . They had observed that the most dominant contribution to the EP/EA comes from the part of the EP involving simple electron attachment ( a i ) and detachment operators ( a i ).…”