A trust-region augmented Hessian implementation for restricted and unrestricted Hartree–Fock and Kohn–Sham methods

We present a trust-region augmented Hessian implementation (TRAH-SCF) for restricted and unrestricted Hartree-Fock Kohn-Sham methods. With TRAH-SCF convergence can always be achieved with tight thresholds, which requires just modest number of iterations. Our benchmark study our illustrative applications focus on open-shell molecules, also antiferromagnetically coupled systems, it is notoriously complicated to converge the Roothaan-Hall self-consistent field (SCF) equations. compare TRAH iterations reach those Pulay's Kolmar's (K) variant direct inversion iterative subspace (DIIS) method analyze obtained SCF solutions. Often finds symmetry-broken solution lower energy than DIIS KDIIS. For calculations, this accompanied by larger spin contamination, i.e. deviation from desired spin-restricted expectation value. However, there are rare cases in KDIIS TRAH. In cases, both may an excited-state determinant solution. calculations negative-gap solutions, standard diverges. If comparable, usually needs more because every new set orbitals level-shifted Newton-Raphson equations solved approximately iteratively means eigenvalue problem. Nevertheless, total runtime still competitive DIIS-based approaches even if extended basis sets employed, illustrated large hemocyanin model complex.