Approximating full configuration interaction with selected configuration interaction and perturbation theory

Selected configuration interaction (CI) calculations and second order perturbation theory are combined to systematically approach the full-CI limit. The resulting algorithm has negligible requirement for memory or disk space, being limited only by available cpu time. Comparison is made to existing full-CI benchmarks (DZ and DZP water, the oxygen atom and its anion, ammonia and the magnesium atom). In all cases the full-CI result is recovered to better than 0.1 kcal/mol.