Although Q-Chem provides defaults for most *$rem* variables, the user will always have to stipulate a few others. For example, in a single point energy calculation, the minimum requirements will be BASIS (defining the basis set) and METHOD (defining the level of theory for correlation and exchange). For example, METHOD=HF invokes a Hartree-Fock calculation, whereas METHOD=CIS specifies a CIS excited-state calculation.

**Example 3.15** Example of minimum *$rem* requirements to run an MP2/6-31G* energy calculation.

$rem BASIS 6-31G* Just a small basis set METHOD mp2 MP2 $end

Alternatively, the level of theory can be specified by two other *$rem* variables, EXCHANGE (defining the level of theory to treat exchange) and CORRELATION (defining the level of theory to treat correlation, if required). For excited states, there is a third *$rem* variable, EOM_CORR, which specifies the level of correlation for the target states.

To invoke DFT calculations, METHOD specifies an exchange-correlation functional (see Section 4.3.17 for the supported functionals).

Supported values for the METHOD keyword for different wave function based correlated methods are listed in Section 5.1; for excited-state calculations, see Section 6.1.

If a wavefunction-based correlation treatment (such as MP2 or CC) is requested by using CORRELATION keyword, HF is taken as the default for EXCHANGE.