Q-Chem is equipped with several standard ECP sets which are specified using the ECP keyword within the $rem block. The built-in ECPs, which are described in some detail at the end of this Chapter, fall into four families:
The Hay-Wadt (or Los Alamos) sets (fit-HWMB and fit-LANL2DZ)
The Stevens-Basch-Krauss-Jansien-Cundari set (fit-SBKJC)
The Christiansen-Ross-Ermler-Nash-Bursten sets (fit-CRENBS and fit-CRENBL)
The Stuttgart-Bonn sets (SRLC and SRSC)
Besides the ones above, a common “def2-ECP” needs to be used with Karlsruhe basis sets for elements Rb-Rn (see Section 8.3).
References and information about the definition and characteristics of most of these sets can be found at the Basis Set Exchange:925
Each of the built-in ECPs comes with a matching orbital basis set for the valence electrons. In general, it is advisable to use these together and, if you select a basis set other than the matching one, Q-Chem will print a warning message in the output file. If you omit the BASIS $rem keyword entirely, Q-Chem will automatically provide the matching one.
The following $rem variable controls which ECP is used: