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# 5.3.8 Semi-Empirical Functionals

(February 4, 2022)

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

• 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 although the double-$\zeta$ 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