Recent Progress in the Variational Orbital Approach to Atomic and Molecular Electronic Structure
Abstract Recent progress in selected configuration interaction (CI) with truncation energy error (SCI TEE) is discussed together with applications. In molecular CI, we take up (i) preselection of huge numbers of configurations and sensitivity analyses, (ii) highlights of SCI TEE applied to H 2 O ground state, and (iii) symmetric dissociation of H 2 O ground state. We describe automatic optimization of atomic orbital bases to within a prescribed complete basis set energy error and an application of it to Ne ground state. New perspectives on the use of optimized orbital bases are briefly outlined. We discuss opportunities for new theory and new predictions in connection with a genuine variational theorem for the Breit Dirac Hamiltonian. We explain the meaning of positive and negative energy orbitals in contrast with positive energy and unphysical N electron states. We conclude with an overview of current and planned work.