Orbital Optimization in Selected Configuration Interaction Methods

We study several approaches to orbital optimization in selected configuration interaction (SCI) plus perturbation theory methods and test them on the ground excited states of three molecules using semistochastic heat-bath method. discuss ways which problem SCI resembles differs from that complete active space self-consistent field. Starting natural orbitals, these divide into classes according how they treat coupling between coefficients parameters, namely uncoupled, fully coupled, quasi-fully coupled methods. demonstrate taking account is crucial for fast convergence recommend two such applications: accelerated diagonal Newton Broyden–Fletcher–Goldfarb–Shanno.