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In this work, we present a one-step second-order converger for state-specific(SS) and state-averaged (SA) complete active space self-consistent field(CASSCF) wave functions. Robust convergence is achieved through steprestrictions using a trust-region augmented Hessian (TRAH) algorithm. To avoidnumerical instabilities, an exponential parametrization of variationalconfiguration parameters is employed, which works with a nonredundantorthogonal complement basis. This is a common approach for SS-CASSCF and isextended to SA-CASSCF wave functions, in this work. Our implementation isintegral direct and based on intermediates that are formulated either in thesparse atomic-orbital or small active molecular-orbital basis. Thus, itbenefits from a combination with efficient integral decomposition techniques,such as the resolution-of-the-identity or the chain-of-spheres for exchangeapproximations. This facilitates calculations on large molecules such as aNi(II) complex with 231 atoms and 5154 basis functions. The runtime performanceof TRAH-CASSCF is competitive with other state-of-the-art implementations ofapproximate and full second-order algorithms. In comparison with asophisticated first-order converger, TRAH-CASSCF calculations usually take moreiterations to reach convergence and, thus, have longer runtimes. However,TRAH-CASSCF calculations still converge reliably to a true minimum even if thefirst-order algorithm fails.

A trust-region augmented Hessian implementation for state-specific and state-averaged CASSCF wave functions