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8.10 Effective Core Potentials (ECPs)

8.10.5 User-Defined ECPs

(May 7, 2024)

Many users will find that the library of built-in ECPs is adequate for their needs. However, if you need to use an ECP that is not built into Q-Chem, you can enter it in much the same way as you can enter a user-defined orbital basis set; see Chapter 8.

To apply a user-defined ECP, you must set the ECP and BASIS keywords in $rem to GEN. You then add a $ecp block that defines your ECP, element by element, and a $basis block that defines your orbital basis set, separating elements by asterisks.

The syntax within the $basis block is described in Chapter 8. The syntax for each record within the $ecp block is as follows:.

For each atom that will bear an ECP:
      Chemical symbol for the atom
      ECP name; the L value for the ECP; number of core electrons removed
      For each ECP component (in the order unprojected, P^0, P^1, …, P^L-1):
            The component name
            The number of Gaussians in the component
            For each Gaussian in the component
                  The power of r ; the exponent ; the contraction coefficient
A sequence of four asterisks (i.e., ****)

Note:  1. All of the information in the $ecp block is case-insensitive. 2. The power of r (which includes the Jacobian r2 factor) must be 0, 1, or 2. 3. If an r0 or r1 term is included you must include the rem keyword “ECP_FIT = TRUE”.

Example 8.10  Optimizing the HF geometry of AlH3 using a user-defined ECP and basis set on Al and the 3-21G basis on H.

   0  1
   H1  Al  r
   H2  Al  r  H1  120.0
   H3  Al  r  H1  120.0  H2  180.0

   r = 1.6

   JOBTYPE    opt   Geometry optimization
   METHOD     hf    Hartree-Fock theory
   ECP        gen   User-defined ECP
   BASIS      gen   User-defined basis
   ECP_FIT    true
   geom_opt_driver optimize

   Stevens_ECP  2  10
   d potential
     1   1.95559  -3.03055
   s-d potential
     0   7.78858   6.04650
     2   1.99025  18.87509
   p-d potential
     0   2.83146   3.29465
     2   1.38479   6.87029

   SP 3  1.00
      0.90110  -0.30377  -0.07929
      0.44950   0.13382   0.16540
      0.14050   0.76037   0.53015
   SP 1  1.00
      0.04874   0.32232   0.47724