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)
Karlsruhe def2-ECPs, for use with the def2 basis sets
References and information about the definition and characteristics of most of
these sets can be found at the Basis Set Exchange:
982
J. Chem. Inf. Model
(2019),
59,
pp. 4814.
Link
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 $rem variable controls which ECP is used:
ECP
ECP
Defines the effective core potential and associated basis set to be used
TYPE:
STRING
DEFAULT:
No ECP
OPTIONS:
General, Gen
User defined. ($ecp keyword required)
Symbol
Use standard ECPs discussed above.
RECOMMENDATION:
ECPs are recommended for first row transition metals and heavier
elements. Consult the reviews for more details.