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^{866, 865} and analytic first
derivatives.^{228} 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.