Subject: ENERGY DIFFERENCES RECONSIDERED

From: "E. Lewars" <[email-masked]>
Date: Fri, 2 May 1997 12:14:34 -0400 (EDT)

1997  May 1

 Hello,

 My response to Joseph Bausch's question about the significance of 1-3 kcal/mol
 (4-13 kJ/mol) generated some good responses.  I was wrong not to distinguish
 between absolute and relative energies. If the question had been "can you
 reliably  calculate absolute energy values like, say, atomization energies to
 1-3 kcal/mol at the MP2/6-31G* level" then _no_ would have been right.
 However, as several people pointed out, _relative_ energies are another matter;
 here errors in absolute energies can cancel, and quite low levels of theory
 give good results, like HF/modest basis with conformational barriers and with
 the relative energies of isomers of similar structure.

  E. Lewars
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