Monday, May 20, 2013

Corrected small basis set Hartree-Fock method for large systems

Contributed by +Jan Jensen 

Steven Bachrach recently highlighted Kruse and Grimme's geometric counterpoise scheme (gCP) - an add on term that corrects for BSSE in the same spirit as Grimme's dispersion correction.  One of the many interesting observations in this paper is that gCP- and dispersion-corrected HF minimal basis set calculations can yield very accurate binding energies.

The paper by Sure and Grimme investigates this particular issue further and presents the so-called HF-3c method.  "3c" stands for the 3 different corrections: gCP, dispersion, and a short-range basis incompleteness (SRB) correction for "systematically overestimated bond lengths for electronegative elements (e.g. N,O,F) when employing small basis sets."  HF-3c uses a minimal basis set for H, C, N, and O and a split valence basis for heavier elements.  In all only 9 empirically adjusted parameters are needed.

Being an HF/minimal basis set calculation the method can be applied to rather large molecules, but will be slower than corresponding semi-empirical calculations on a single core.  However, the HF-3c can of course be run in parallel. HF-3c interaction energies computed for the S22, S66, and X40 datasets are of similar accuracy to PM6-DH2 and significantly better than PM6.

HF-3c geometry optimizations and vibrational frequency calculations are found to significantly more numerically stable and offer more accurate structures (including protein structures) than PM6, as implemented in MOPAC2012.  HF-3c thus offers an attractive alternative when MOPAC2012 is giving imaginary frequencies.  If HF-3c can produces accurate transition state geometries, the method might also be a viable alternative to QM/MM calculations.

HF-3c is implemented in the ORCA package, which is available free of charge.

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