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# 6.6.9 Dual-Basis MP2

(February 4, 2022)

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 energies1072, 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.