Virtual Orbital Many-Body Expansions: A Possible Route towards the Full Configuration Interaction Limit

It is demonstrated how full configuration interaction (FCI) results in extended basis sets may be obtained to within sub-kJ/mol accuracy by decomposing the energy in terms of many-body expansions in the virtual orbitals of the molecular system at hand. This extension of the FCI application range lends itself to two unique features of the current approach, namely, that the total energy calculation can be performed entirely within considerably reduced orbital subspaces and may be so by means of embarrassingly parallel programming. Facilitated by a rigorous and methodical screening protocol and further aided by expansion points different from the Hartree-Fock solution, all-electron numerical results are reported for H₂O in polarized core-valence basis sets ranging from double-ζ (10 e, 28 o) to quadruple-ζ (10 e, 144 o) quality.