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A perturbation-based super-CI approach for the orbital optimization of a CASSCF wave function

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Kollmar,  Christian
Research Department Neese, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Sivalingam,  Kantharuban
Research Group Wennmohs, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Helmich-Paris,  Benjamin
Research Group Helmich-Paris, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Neese,  Frank
Research Department Neese, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Citation

Kollmar, C., Sivalingam, K., Helmich-Paris, B., Angeli, C., & Neese, F. (2019). A perturbation-based super-CI approach for the orbital optimization of a CASSCF wave function. Journal of Computational Chemistry, 40(14), 1463-1470. doi:10.1002/jcc.25801.


Cite as: http://hdl.handle.net/21.11116/0000-0004-3FBC-3
Abstract
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.