9.5 ECPs and Electron Correlation

The ECP package is integrated with the electron correlation package and it is therefore possible to apply any of Q-Chem’s post-Hartree-Fock methods to systems in which some of the atoms may bear pseudopotentials. Of course, the correlation energy contribution arising from core electrons that have been replaced by an ECP is not included. In this sense, correlation energies with ECPs are comparable to correlation energies from frozen-core calculations. However, the use of ECPs effectively removes both core electrons and the corresponding virtual (unoccupied) orbitals.

Any of the local, gradient-corrected and hybrid functionals discussed in Chapter 5 may be used and you may also perform ECP calculations with user-defined hybrid functionals. In a DFT calculation with ECPs, the exchange-correlation energy is obtained entirely from the non-core electrons. This will be satisfactory if there are no chemically important cores/valence effects but may introduce significant errors if not, particularly if you are using a “large-core” ECP.

Example 9.208 Optimization of the structure of Se $_{8}$ 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
   JOBTYPE       sp        Single-point energy
   METHOD        mp2       MP2 correlation energy
   ECP           fit-lanl2dz   Hay-Wadt ECP and basis
   SCF_GUESS     read      Read in the MOs
$end