Automatic Selection of an Active Space for Calculating Electronic Excitation Spectra by MS-CASPT2 or MC-PDFT

Abstract

Multireference methods such as multistate complete active space second-order perturbation theory (MS-CASPT2) and multiconfiguration pair-density functional theory (MC-PDFT) can be very accurate, but they have the disadvantage that they are not black-box methods, and finding a good active space for the reference wave function often requires nonsystematic procedures based on intimate knowledge of the system under study. In this paper, we propose a scheme called the ABC scheme, which is a three-step calculation using three parameters, for automatic selection (without looking at the orbitals and without using any knowledge of the specific system at hand) of the active space (including both the size of the active space and the orbitals in the active space) for MS-CASPT2 or MC-PDFT calculations, and we report tests of the scheme on the first five excitation energies for a set of ten doublet radicals. The results show that the ABC scheme is very successful for both MS-CASPT2 and MC-PDFT for all ten systems considered here.

Second-order MCSCF optimization revisited. I. Improved algorithms for fast and robust second-order CASSCF convergence

“…10,15,66,67 However, despite this formally simple ansatz, much experience and many trials are often necessary until a reasonable active space is found. New tools [68][69][70][71] have recently been proposed for simplifying the construction and selection of the active space and driving the CASSCF method closer to a black-box application, but this problem is still not yet generally solved.…”

Complete active space analysis of a reaction pathway: Investigation of the oxygen–oxygen bond formation

“…Various approaches have been suggested for choosing active spaces. [ 49–64 ] In contrast to that, only a few approaches for a consistent complete active space analysis of reaction pathways for nontrivial systems have been published. The atomic valence active space (AVAS) method [ 65 ] has been introduced to generate the initial guess for CASSCF calculations by maximizing the given atomic valence character, and its application to one example, namely the Fenton reaction, was demonstrated, however using complete active space configuration interaction instead of CASSCF.…”

Emerging DFT Methods and Their Importance for Challenging Molecular Systems with Orbital Degeneracy

“…Since the latter magnitude is built from a multi-determinantal wave function, it includes by definition all many-body effects qualitatively. The selection of the active space is done automatically [38] and analytical gradients are also available [39], thus, perfectly complementing historical attempts to merge wave function and DFT methods for strongly correlated systems [40][41][42].…”