There are several types of calculation that can be performed within Q-Chem using *two* atomic orbital basis sets instead of just one as we have been assuming in this chapter so far. Such calculations are said to involve *dual* basis sets. Typically iterations are performed in a smaller, primary, basis, which is specified by the *$rem* keyword BASIS2. Examples of calculations that can be performed using dual basis sets include:

An improved initial guess for an SCF calculation in the large basis. See Section 4.4.5.

Dual basis self-consistent field calculations (Hartree-Fock and density functional theory). See discussion in Section 4.7.

Density functional perturbative corrections by “triple jumping”. See Section 4.8.2.

Dual basis MP2 calculations. See discussion in Section 5.5.1.

BASIS2

Defines the (small) second basis set.

TYPE:

STRING

DEFAULT:

No default for the second basis set.

OPTIONS:

Symbol

Use standard basis sets as for BASIS.

BASIS2_GEN

General BASIS2

BASIS2_MIXED

Mixed BASIS2

RECOMMENDATION:

BASIS2 should be smaller than BASIS. There is little advantage to using a basis larger than a minimal basis when BASIS2 is used for initial guess purposes. Larger, standardized BASIS2 options are available for dual-basis calculations as discussed in Section 4.7 and summarized in Table 4.4.

In addition to built-in basis sets for BASIS2, it is also possible to enter user-defined second basis sets using an additional *$basis*2 input section, whose syntax generally follows the *$basis* input section documented above in Section 7.4.