8 Basis Sets and Effective Core Potentials

8.11 Built-In ECPs

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 EMSL site of the Pacific Northwest National Laboratory:EMSL

http://www.emsl.pnl.gov/forms/basisform.html

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
       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. Consul the reviews for more details.

8.11.0.1 Combining ECPs

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 (equivalently) “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 “****”. There is also a built-in combination that can be invoked specifying ECP = fit-LACVP. It assigns automatically 6-31G* or other suitable type basis sets for atoms H–Ar, while uses fit-LANL2DZ for heavier atoms.

8.11.0.2 Examples

Example 8.6  Computing the HF/fit-LANL2DZ energy of AgCl at a bond length of 2.4 Å.

$molecule
   0  1
   Ag
   Cl  Ag  r

   r  =  2.4
$end

$rem
   METHOD     hf        Hartree-Fock calculation
   ECP        fit-lanl2dz   Using the Hay-Wadt ECP
   BASIS      lanl2dz   And the matching basis set
$end

Example 8.7  Computing the single point energy of HI with B3LYP/def2-SV(P) (using def2-ECP for I).

$molecule
   0 1
   H    0.0    0.0    0.0
   I    0.0    0.0    1.5
$end

$rem
   METHOD           b3lyp
   BASIS            def2-sv(p)
   ECP              def2-ecp
   SYMMETRY         false
   SYM_IGNORE       true
   THRESH           14
   SCF_CONVERGENCE  8
$end

Example 8.8  Optimization of the structure of Se8 using HF/fit-LANL2DZ, followed by a single-point energy calculation at the MP2/fit-LANL2DZ level.

$molecule
   0  1
   x1
   x2   x1  xx
   Se1  x1  sx  x2  90.
   Se2  x1  sx  x2  90.  Se1  90.
   Se3  x1  sx  x2  90.  Se2  90.
   Se4  x1  sx  x2  90.  Se3  90.
   Se5  x2  sx  x1  90.  Se1  45.
   Se6  x2  sx  x1  90.  Se5  90.
   Se7  x2  sx  x1  90.  Se6  90.
   Se8  x2  sx  x1  90.  Se7  90.

   xx = 1.2
   sx = 2.8
$end

$rem
   JOBTYPE    opt
   METHOD     hf
   ECP        fit-lanl2dz
$end

@@@

$molecule
   read
$end

$rem
   METHOD        mp2       MP2 correlation energy
   ECP           fit-lanl2dz   Hay-Wadt ECP and basis
   SCF_GUESS     read      Read in the MOs
$end

Example 8.9  Computing the HF geometry of CdBr2 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.

$molecule
   0  1
   Cd
   Br1  Cd  r
   Br2  Cd  r  Br1  180.0

   r = 2.4
$end

$rem
   JOBTYPE    opt   Geometry optimization
   METHOD     hf    Hartree-Fock theory
   ECP        gen   Combine ECPs
   BASIS      gen   Combine basis sets
   PURECART   1     Use pure d functions
$end

$ecp
   Cd
   srsc
   ****
   Br
   srlc
   ****
$end

$basis
   Cd
   srsc
   ****
   Br
   srlc
   ****
$end