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

Kreplin, David A., Knowles, Peter J. and Werner, Hans-Joachim 2019. Second-order MCSCF optimization revisited. I. Improved algorithms for fast and robust second-order CASSCF convergence. Journal of Chemical Physics 150 (19) , 194106. 10.1063/1.5094644

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Abstract

A new improved implementation of the second-order multiconfiguration self-consistent field optimization method of Werner and Knowles [J. Chem. Phys. 82, 5053 (1985)] is presented. It differs from the original method by more stable and efficient algorithms for minimizing the second-order energy approximation in the so-called microiterations. Conventionally, this proceeds by alternating optimizations of the orbitals and configuration (CI) coefficients and is linearly convergent. The most difficult part is the orbital optimization, which requires solving a system of nonlinear equations that are often strongly coupled. We present a much improved algorithm for solving this problem, using an iterative subspace method that includes part of the orbital Hessian explicitly, and discuss different strategies for performing the uncoupled optimization in a most efficient manner. Second, we present a new solver in which the orbital-CI coupling is treated explicitly. This leads to quadratic convergence of the microiterations but requires many additional evaluations of reduced (transition) density matrices. In difficult optimization problems with a strong coupling of the orbitals and CI coefficients, it leads to much improved convergence of both the macroiterations and the microiterations. Third, the orbital-CI coupling is treated approximately using a quasi-Newton approach with Broyden–Fletcher–Goldfarb–Shanno updates of the orbital Hessian. It is demonstrated that this converges almost as well as the explicitly coupled method but avoids the additional effort for computing many transition density matrices. The performance of the three methods is compared for a set of 21 aromatic molecules, an Fe(ii)-porphine transition metal complex, as well as for the [Cu2O2(NH3) 6]2+, FeCl3, Co2(CO)6C2H2, and Al4O2 complexes. In all cases, faster and more stable convergence than with the original implementation is achieved.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Advanced Research Computing @ Cardiff (ARCCA)
Publisher: American Institute of Physics (AIP)
ISSN: 0021-9606
Date of First Compliant Deposit: 21 May 2019
Date of Acceptance: 25 April 2019
Last Modified: 22 May 2020 00:17
URI: http://orca.cf.ac.uk/id/eprint/122722

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