Q-Chem 5.0 User’s Manual

8.5 ECPs and Density Functional Theory

Q-Chem’s ECP 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 ECPs, 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.