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.
Dual basis MP2 calculations. See discussion in Section 5.5.1.
Defines the (small) second basis set.
No default for the second basis set.
Use standard basis sets as for BASIS.
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 $basis2 input section, whose syntax generally follows the $basis input section documented above in Section 7.4.