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A perturbation‐based super‐CI approach for the orbital optimization of a CASSCF wave function
Author(s) -
Kollmar Christian,
Sivalingam Kantharuban,
HelmichParis Benjamin,
Angeli Celestino,
Neese Frank
Publication year - 2019
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.25801
Subject(s) - wave function , perturbation (astronomy) , physics , perturbation theory (quantum mechanics) , computational chemistry , atomic physics , quantum mechanics , chemistry
A perturbation theory‐based algorithm for the iterative orbital update in complete active space self‐consistent‐field (CASSCF) calculations is presented. Following Angeli et al. (J. Chem. Phys. 2002, 117, 10525), the first‐order contribution of singly excited configurations to the CASSCF wave function is evaluated using the Dyall Hamiltonian for the determination of a zeroth‐order Hamiltonian. These authors employ an iterative diagonalization of the first‐order density matrix including the first‐order correction arising from single excitations, whereas the present approach uses the single‐excitation amplitudes directly for the construction of the exponential of an anti‐Hermitian matrix resulting in a unitary matrix which can be used for the orbital update. At convergence, the single‐excitation amplitudes vanish as a consequence of the generalized Brillouin's theorem. It is shown that this approach in combination with direct inversion of the iterative subspace (DIIS) leads to very rapid convergence of the CASSCF iteration procedure. © 2019 Wiley Periodicals, Inc.