Semiempirical methods

Semiempirical methods are usually simplified versions of the Hartree-Fock method where certain integrals have been skipped or approximated and empirical parameters introduced. Semiempirical methods are designed to be computationally efficient and the empirical parameters have been fitted to make them semi-accurate. As the accuracy of these methods is limited and they are nowhere near as robust as methods based on density functional theory or wavefunction theory, these methods have gone a little out of fashion. It is worth keeping these methods in mind, however, as their low computational cost means one can do geometry optimizations of very large molecules very quickly or much faster molecular dynamics than with DFT.

MNDO, AM1 and PM3 methods

! MNDO

! AM1

! PM3

ZINDO methods

HF-3c

Grimme's 3-corrected Hartree-Fock method (HF-3c) is not a traditional semiempirical method but may be an attractive alternative to the before-mentioned methods. The idea is to correct for some of the systematic deficiencies of a small basis Hartree-Fock calculation (instead of approximating HF) and use as a very fast QM method. This involves correcting for basis set superposition error by the geometric counterpoise scheme (gCP), correcting for dispersion by Grimme's well established D3 approach and a correction for short-range basis incompleteness (9 empirical parameters are used overall). As no integrals are skipped, the HF-3c method will be more costly than a traditional semiempirical method, but the advantage is the results should be much more robust. It's also in many ways a better alternative to a minimal basis DFT method as there is no numerical integration involved. HF-3c has been defined for elements H-Xe with ECPs automatically used for the heavier elements. See more in chapter 6.2.2.13 in the manual or the original publication.

The HF-3c method can be used with a simple keyword:

! HF-3c

which corresponds to the full inputline:

! HF MINIX D3BJ GCP(HF/MINIX) PATOM)

Integral handling is automatically set to Conventional which means the 2-electron integrals are stored in memory which is beneficial here. To use Direct mode instead (recalculate integrals every time), this can be specified like this:

%scf

SCFMode Direct

end