5.3.8 Semi-Empirical Functionals

The following 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. ¹²⁸⁵ 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, ⁴⁷¹ Grimme S. et al.
  J. Chem. Phys.
  (2015), 143, pp. 054107. Link 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. • Analytic frequencies are available for these DFT-3c methods, with derivatives of the “3c” term computed numerically. In most cases this should be invisible to the user; see Section 5.3.8.2 for job control settings related to semi-numerical calculation of the Hessian.