Q-Chem 4.4 User’s Manual

8.4 Pseudopotentials and Density Functional Theory

Q-Chem’s pseudopotential package and DFT package are tightly integrated and facilitate the application of advanced density functionals to molecules containing heavy elements. Any of the local, gradient-corrected and hybrid functionals discussed in Chapter 4 may be used and you may also perform ECP calculations with user-defined hybrid functionals.

In a DFT calculation with pseudopotentials, the exchange-correlation energy is obtained entirely from the non-core electrons. This will be satisfactory if there are no chemically important core-valence effects but may introduce significant errors, particularly if you are using a “large-core” ECP.

Q-Chem’s default quadrature grid is SG-1 (see section 4.4.5) which was originally defined only for the elements up to argon. In Q-Chem 2.0 and above, the SG-1 grid has been extended and it is now defined for all atoms up to, and including, the actinides.

8.4.1 Example

Example 8.188  Optimization of the structure of XeF$_{5}^{+}$ using B3LYP theory and the ECPs of Stevens and collaborators. Note that the BASIS keyword has been omitted and, therefore, the matching SBKJC orbital basis set will be used.

   1  1
   F1  Xe  r1
   F2  Xe  r2  F1  a
   F3  Xe  r2  F1  a  F2  90
   F4  Xe  r2  F1  a  F3  90
   F5  Xe  r2  F1  a  F4  90

   r1  =  2.07
   r2  =  2.05
   a   =  80.0

   JOBTYP     opt
   METHOD     b3lyp
   ECP        sbkjc