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Users can also request a customized density functional consisting of any linear
combination of exchange and/or correlation functionals available in
Q-Chem. A “general” density functional of this sort is requested by
setting EXCHANGE = GEN and then specifying the functional by
means of an *$xc_functional* input section consisting of one line for each
desired exchange (X) or correlation (C) component of the functional, and having
the format shown below.

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

Each line requires three variables: X or C to designate whether this is an exchange or correlation component; the symbolic representation of the functional, as would be used for the EXCHANGE or CORRELATION keywords variables as described above; and a real number coefficient for each component. Note that Hartree-Fock exchange can be designated either as “X" or as “K". Examples are shown below.

$molecule 0 1 O H1 O oh H2 O oh H1 hoh oh = 0.97 hoh = 120.0 $end $rem EXCHANGE gen CORRELATION none BASIS g3large ! recommended for high accuracy THRESH 14 ! and better convergence $end $xc_functional X Becke 0.726 X S 0.0966 C PK06 1.0 K 0.1713 $end

$molecule 0 1 O H1 O oh H2 O oh H1 hoh oh = 0.97 hoh = 120.0 $end $rem EXCHANGE gen CORRELATION none BASIS g3large ! recommended for high accuracy THRESH 14 ! and better convergence $end $xc_functional X BR89 0.846 C B94hyb 1.0 K 0.154 $end