Dual-basis calculations are controlled with the following *$rem*.
DUAL_BASIS_ENERGY turns on the dual-basis approximation. Note that
use of BASIS2 without DUAL_BASIS_ENERGY *only* uses
basis set projection to generate the initial guess and does not invoke the
dual-basis approximation (see Section 4.4.5). OVPROJECTION is used as the default
projection mechanism for dual-basis calculations; it is not recommended that
this be changed. Specification of SCF variables
(*e.g.*, THRESH) will apply to calculations in both basis sets.

DUAL_BASIS_ENERGY

Activates dual-basis SCF (HF or DFT) energy correction.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

Analytic first derivative available for HF and DFT (see JOBTYPE)
Can be used in conjunction with MP2 or RI-MP2
See BASIS, BASIS2, BASISPROJTYPE

RECOMMENDATION:

Use dual-basis to capture large-basis effects at smaller basis cost.
Particularly useful with RI-MP2, in which HF often dominates. Use only proper
subsets for small-basis calculation.