5.3 Overview of Available Functionals

5.3.6 User-Defined Density Functionals

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.

Example 5.2  Q-Chem input for H2O with the B3tLap functional.

$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

Example 5.3  Q-Chem input for H2O with the BR89B94hyb functional.

$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