# 3.5.4 User-Defined Exchange-Correlation Density Functionals ($xc_functional) If the keyword EXCHANGE = GEN then a DFT calculation will be performed using a user-specified combination of exchange and correlation functional(s), as described in Chapter 4. Custom functionals of this sort can be constructed as any linear combination of exchange and/or correlation functionals that are supported by Q-Chem; see Section 5.4 for a list of supported functionals. The format for the$xc_functional input section is the following:

$xc_functional X exchange_symbol coefficient X exchange_symbol coefficient ... C correlation_symbol coefficient C correlation_symbol coefficient ... K coefficient$end


Note:  The coefficients must be real numbers.