Gaussian-N composite thermochemical methods

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Quote:Mernst May 9th 11:27 am
This is one of my long-term interests with NWChem and other quantum chemistry packages that I am writing a job driver for. I did some preliminary tests last year. It is tricky to reproduce the Gn methods exactly in packages other than Gaussian because there is an implicit dependency on Gaussian's orbital freezing conventions for post-HF calculations.

I got a <100 microHartree (~1/16 kcal) agreement between NWChem, GAMESS, and Gaussian for single point energies on identical geometries last time I checked, but it took attention to the cartesian vs. spherical handling of basis sets. Of DFTs, B3LYP is a term of art, implementation dependent, and best avoided for reproducibility. wB97X-D, however, is your friend.

It would be best if you could get the special basis sets submitted to the EMSL Basis Set Exchange. I already have a Python API to get basis sets out of the BSE, store them in a SQLite database, and offer programmable access to them in NWChem, GAMESS-US, and Gaussian94 formats.

Will do. Curtiss delivered them as complete through the third row, so any G3(etc) using them can deliver accordingly.

I don't have access to Gaussian.

Ditto. No great loss, although the completeness and ease of use are impressive, if only in comparison to GAMESS.

I have been calibrating my work from the original publications, the occasional Gaussian log file that you can find on the web, and the composite thermochemical methods implemented in GAMESS-US and Psi4. The GAMESS implementations are quirky. You should search the GAMESS user group archives before diving in with them.

The GAMESS G3(MP2) implementation is good in its delta-Hf values but particular data sets in the source code are irreproducible. Lacking RO-CCSD(T), GAMESS could not have generated the necessary total energies for non-singlet atomic species and the author is not forthcoming about their origin. I'd hoped to extend it to 3rd row elements as per Curtiss, Redfern,, but decided it would be like passing a twisted sardine can lid wasn't worth the trouble. GAMESS has incompletenesses and defects which have lain unaddressed for a decade and will probably never be fixed.

So here I am.

The RTDB approach is cleaner if you stick with NWChem, and of course it is advantageous to be able to submit a single job containing all the phases if you go through a job queue.

I want RTDB support that allows me to specify a geometry normally and write little beyond this:

print none
import g3mp2
(E0, H0, H298, G0, dHf0, dHf298) = g3mp2()
print "Energy(0K) = ", E0
print "Enthalpy(0K) = ", H0
print "Enthalpy(298K) = ", H298
print "Gibbs Free Energy= ", G0
print "Heat of Formation(0K) = ",dHf0
print "Heat of Formation(298K)= ",dHf298
task python

That, IMHO, would utterly rock! lead to clean implementations of G4(mumble) and assorted ccCA-? composite methods, perhaps even a CBS-QB3 which is often cited in the same paper as G4 results.

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