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 4.4.4.
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.
Dual basis MP2 calculations. See discussion in Section 6.6.1.
In addition to built-in basis sets for BASIS2, it is also possible to enter user-defined second basis sets using an additional $basis2 input section, whose syntax generally follows the $basis input section documented above in Section 8.4.