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(May 16, 2021)

The dual-basis approximation can also be used for the reference energy of a
correlated second-order Møller-Plesset (MP2)
calculation.
^{
1040
}
J. Chem. Phys.

(2006),
125,
pp. 074108.
Link
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^{
1039
}
J. Chem. Theory Comput.

(2009),
5,
pp. 1560.
Link
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
^{
270
}
Mol. Phys.

(2007),
105,
pp. 2731.
Link
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.