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5.3 Overview of Available Functionals

5.3.8 Semi-Empirical Functionals

(December 20, 2021)

The following semi-empirical methods are available in Q-Chem:

  • HF-3c: It is an HF-based semi-empirical method which is used with a minimal basis called MINIX. All the elements from H to Xe are supported. For the elements from Rb (Z=37) to Xe (Z=54), def2-ECP must be used. 1100 Sure R., Grimme S.
    J. Comput. Chem.
    (2013), 34, pp. 1672.
    Link

  • PBEh-3c: It is a DFT based method which is used with def2-mSVP basis set.This is available for all the elements from H to Rn. For the elements from Rb (Z=37) to Rn (Z=86), def2-ECP has to be used. It is a more accurate method than HF-3c as it has correlation, 393 Grimme S. et al.
    J. Chem. Phys.
    (2015), 143, pp. 054107.
    Link
    although the double-ζ basis set makes it more expensive for large molecules.

Note:  The HF-3c and PBEh-3c methods were parameterized for use with the aforementioned basis sets. Although Q-Chem does not enforce this, other basis sets should not be used unless there is a good reason for doing so, and only then with caution as the results may not match published accuracy.

Example 5.4  Q-Chem input for HF-3c method.

$rem
 method  hf-3c
 basis   minix
 ecp     def2-ecp
$end

$molecule
-1 1
        I         0.6539056222    0.0300979939    0.0000000000
        O        -2.8593081518    0.0835488510    0.0000000000
        H        -1.8947008157    0.2596704547    0.0000000000
        H        -2.8641221666   -0.8778240298    0.0000000000
$end

View output

Example 5.5  Q-Chem input for PBEh-3c method.

$molecule
   0 1
   O  -1.551007  -0.114520   0.000000
   H  -1.934259   0.762503   0.000000
   H  -0.599677   0.040712   0.000000
   O   1.350625   0.111469   0.000000
   H   1.680398  -0.373741  -0.758561
   H   1.680398  -0.373741   0.758561
$end

$rem
   METHOD           PBEh-3c
   BASIS            def2-mSVP
   XC_GRID          000099000590
   INCDFT           0
   INCFOCK          0
   SCF_CONVERGENCE  8
   THRESH           14
   SYMMETRY         false
   SYM_IGNORE       true
$end

View output