6.6.9 Dual-Basis MP2

The successful computational cost speedups of the previous sections often leave the cost of the underlying SCF calculation dominant. The dual-basis method provides a means of accelerating the SCF by roughly an order of magnitude, with minimal associated error (see Section 4.7). This dual-basis reference energy may be combined with RI-MP2 calculations for both energies^{1072, 1071} and analytic first derivatives.²⁷⁵ In the latter case, further savings (beyond the SCF alone) are demonstrated in the gradient due to the ability to solve the response ($Z$-vector) equations in the smaller basis set. Refer to Section 4.7 for details and job control options.