8.13.1 Overview

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:.

$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, ${\widehat{P}}_0$, ${\widehat{P}}_1$, …, ${\widehat{P}}_{L-1}$):
            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 $r^{\mathrm{2}}$ factor) must be 0, 1, or 2. 3. If an $r^{\mathrm{0}}$ or $r^{\mathrm{1}}$ term is included you must include the rem keyword “ECP_FIT = TRUE”.