Restricted-Open-Shell G4(MP2)-Type Procedures

In the present study, we have reformulated
the G4­(MP2) and G4­(MP2)-6X
procedures for use with a restricted-open-shell (RO) formalism. We
find that the resulting ROG4­(MP2) and ROG4­(MP2)-6X procedures generally
perform comparably to the original unrestricted (U) variants, including
their performance on radicals. Our analysis suggests that this is
due mainly to the inclusion of empirical parameters that overcome
the slightly less good performance of the U variants. However, a major
practical advantage of ROG4­(MP2) and ROG4­(MP2)-6X is that they can
be used in a wider range of computational chemistry software packages
than the U analogs. We have demonstrated the importance of this aspect
with a large-scale ROG4­(MP2)-6X computation for the dissociation of
the dodecahedryl dimer (C20H19)2.