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 Of course, the correlation energy contribution arising from core electrons that have been replaced by an ECP is *not* included.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 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