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# 8.12.1 Introduction

(February 4, 2022)

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