9.4 User-Defined ECPs

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.

9.4.1 Job Control for User-Defined ECPs

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:.

$ecp
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, $\hat{P}_0$ , $\hat{P}_1$ , , $\hat{P}_{L-1}$
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., ****)
$end

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

9.4.2 Example

Example 9.207 Optimizing the HF geometry of AlH $_{3}$ using a user-defined ECP and basis set on Al and the 3-21G basis on H.

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

   r = 1.6
$end

$rem
   JOBTYPE    opt   Geometry optimization
   METHOD     hf    Hartree-Fock theory
   ECP        gen   User-defined ECP
   BASIS      gen   User-defined basis
   ECP_FIT = TRUE
$end

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

$basis
   Al
   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
****
   H
   3-21G
****
$end