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 (HWMB and LANL2DZ)
The Stevens-Basch-Krauss-Jansien-Cundari set (SBKJC)
The Christiansen-Ross-Ermler-Nash-Bursten sets (CRENBS and CRENBL)
The Stuttgart-Bonn sets (SRLC and SRSC)
References and information about the definition and characteristics of most of these sets can be found at the EMSL site of the Pacific Northwest National Laboratory :
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 $rem variable controls which ECP is used:
Defines the effective core potential and associated basis set to be used
User defined. ($ecp keyword required)
Use standard pseudopotentials discussed above.
Pseudopotentials are recommended for first row transition metals and heavier elements. Consul the reviews for more details.
If you wish, you can use different ECP sets for different elements in the system. This is especially useful if you would like to use a particular ECP but find that it is not available for all of the elements in your molecule. To combine different ECP sets, you set the ECP and BASIS keywords to “Gen” or “General” and then add a $ecp block and a $basis block to your input file. In each of these blocks, you must name the ECP and the orbital basis set that you wish to use, separating each element by a sequence of four asterisks. There is also a built-in combination that can be invoked specifying “ECP=LACVP”. It assigns automatically 6-31G* or other suitable type basis sets for atoms H-Ar, while uses LANL2DZ for heavier atoms.
Example 8.185 Computing the HF/LANL2DZ energy of AgCl at a bond length of 2.4 .
Cl Ag r
r = 2.4
METHOD hf Hartree-Fock calculation
ECP lanl2dz Using the Hay-Wadt ECP
BASIS lanl2dz And the matching basis set
Example 8.186 Computing the HF geometry of CdBr using the Stuttgart relativistic ECPs. The small-core ECP and basis are employed on the Cd atom and the large-core ECP and basis on the Br atoms.
Br1 Cd r
Br2 Cd r Br1 180
r = 2.4
JOBTYPE opt Geometry optimization
METHOD hf Hartree-Fock theory
ECP gen Combine ECPs
BASIS gen Combine basis sets
PURECART 1 Use pure d functions