The dual-basis approximation can also be used for the reference energy of a
correlated second-order Møller-Plesset (MP2)
calculation.^{Steele:2006a, Steele:2009} When activated, the dual-basis HF
energy is first calculated as described above; subsequently, the MO
coefficients and orbital energies are used to calculate the correlation energy
in the large basis. This technique is particularly effective for RI-MP2
calculations (see Section 6.6), in which the cost of the
underlying SCF calculation often dominates.

Furthermore, efficient analytic gradients
of the DB-RI-MP2 energy have been developed^{DiStasio:2007a} and added to Q-Chem.
These gradients allow for the optimization of molecular structures with RI-MP2 near the
basis set limit. Typical computational savings are on the order of 50% (aug-cc-pVDZ) to 71% (aug-cc-pVTZ).
Resulting dual-basis errors are only 0.001 Å in molecular structures and are, again,
significantly less than use of a smaller basis set alone.