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 4.5.5.

Dual basis self-consistent field calculations (Hartree-Fock and density functional theory). See discussion in Section 4.8.

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

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 4.8 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.