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.