Subject: re:Calculating heats of formation

From: Russ Johnson <[email-masked]>
Date: Tue, 01 Aug 2000 19:19:43 -0400

Hello,
	I would like to mention a resource that may help a little. The NIST
computational chemistry comparison and benchmark database (CCCBDB)(at
http://srdata.nist.gov/cccbdb ) allows a user to compare enthalpies of
reaction and enthalpies of atomization for ab initio methods (about 18
methods and 9 basis sets). It has results for about 600 small (6 or fewer
heavy atoms). It does not (yet) have any calculations using the STO-3G
basis set or any molecule as large as an azo species. A semi-empirical
method may be a better choice for heats of formation rather than an ab
initio method with a minimal basis set. The CCCBDB has results from
semiempirical methods (AM1, PM3) if you would like to compare to some
calculated and experimental enthalpies  (or experimental and calculated
geometries) at those levels of theory.

Dr. Russell D. Johnson III
Research Chemist
National Institute of Standards and Technology
Computational Chemistry Group
100 Bureau Drive, Stop 8380
Gaithersburg, MD 20899-8380
voice 301+975-2513     fax  301+869-4020
email: [email-masked]

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From: "Oscar N. Ventura" <[email-masked]>
Date: Tue, 1 Aug 2000 22:26:11 -0300 (GRNLNDST)

Just my $ 0.02:

The method of choice for calculating enthalpies of formation of any
reasonably large molecule is DFT. Provided that one can write an isodesmic
reaction for the molecule of interest, basis sets as modest as 6-31G(d)
can give good results. If no such reaction can be written, then one can
use the atomization reactions. In this case, CCSD(T) with Dunning basis
would be preferable (provided that the calculations could be done) and
corrections for core correlation, spin-orbit and relativistic effects must
be included. Look up papers by Martin, Bauschlicher, Lee and others.

BTW, IMHO a STO-3G calculation will give meaningless results.

Oscar.

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Prof.Dr.Oscar N. Ventura             Tel.+(5982)9248396 Fax +(5982)9241906
MTC-Lab,Facultad de Quimica          MTC-Lab  http://bilbo.edu.uy/MTC-Lab.html 
Universidad de la Republica
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Montevideo 11800, URUGUAY            necessarily those of my employer          

"Violence is the last refuge of the incompetent." --- Salvor Hardin 
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