G4(MP2)-6X: A cost-effective improvement to G4(MP2)

G4(MP2)-6X is developed as a composite procedure with a cost comparable to that of G4(MP2) but performance approaching that of G4. The new procedure is a variant of G4(MP2) that employs BMK/6-31+G(2df,p) geometries and has six additional scaling factors for the correlation energy components. The scaling factors and HLC parameters are optimized using the new E2 set of 526 energies, representing thermochemical properties, reaction energies and barriers, and weak interactions. G4(MP2)-6X achieves a mean absolute deviation (MAD) from benchmark values of 3.64 kJ mol-1 for the E2 set, compared with 4.42 kJ mol-1 for G4(MP2). For the E0 set of 148 energies, G4(MP2)-6X gives an MAD of 3.43 kJ mol-1, compared with 3.22 kJ mol-1 for G4 and 4.03 kJ mol-1 for G4(MP2). The new G4(MP2)-6X procedure thus uses extra parametrization to provide a G4-type performance without incurring G4-type computational costs