Unification of the W1X and G4(MP2)-6X Composite Protocols

We have devised the
composite procedures WG and WGh to unify the
W1X and the (computationally more economical) G4­(MP2)-6X protocols.
The WG procedure employs a combination of MP2, MP2-F12, CCSD-F12b,
and CCSD­(T) to approximate the all-electron scalar-relativistic CCSD­(T)/CBS
energy. In addition, it incorporates features such as the scaling
of the energy components and an empirical “higher-level-correction”
term. The WGh protocol represents a somewhat more economical variant
of WG with partial removal of diffuse functions. Our benchmark shows
that, in general, both WG and WGh have similar performance to that
for W1X-2, with WGh (predictably) performing somewhat less well for
electron affinities. In terms of computational efficiency, WG is approximately
an order of magnitude less costly than W1X-2, while WGh gives not
only a further slight savings in computer time but also a notably
reduced disk requirement.