Subject: Re: CCL:1-3 kcal/mol

From: <[email-masked]>
Date: Fri, 2 May 1997 10:08:17 -0400 (EDT)

My two points regarding quantum calculation convergency:
1. The standard way to test reliability of calculations: 
	i) do calculations with different basis sets(with increasing
quality
	ii) do calculations starting from HF(SCF) then
MP2,MP3,MP4....CCSD(T)... 
	iii) then you will know the trend of convergency. For large
system, single point calculations at the correlation level are
recommended. You must do this way if you are uneasy about your
calculations reliability.  
2. convergency varies with chemical systems. It is rather easy for well
behavoired molecules (closed shell main elements molecules +
nonconjugated). Care should be allway excised for open shell molecules
or molecules with transition metals.
	I agree with the point that the relative energies are more
reliable than absolute energy. To make it more clear, I would say
that it is pretty reliable to obtain relative energies between
isomers with similar bonds(especially for rotamers:)).

=================================================================
Dr. Buyong Ma             [email-masked]
Computational Center for Molecular Structure and Design
Department of Chemistry
University of Georgia
Athens, Georgia 30602 USA            Voice (706) 542-2044
=================================================================

On Thu, 24 Apr 1997, Joseph W. Bausch wrote:

> 
> 
> My group has been using Gaussian 94 to optimize the geometries of a variety
> of polyboranes and heteroatom substituted polyboranes.  We usually go up to
> the MP2(FULL)/6-31G* level and calculate the relative energies of isomeric
> systems at this level with a zero point energy correction from a HF/6-31G*
> level calculation.  We have found a handful of systems where two tautomers
> exist on the potential energy surface differing only by ~ 1-3 kcal/mol.
> Most of these tautomeric pairs are relatively similar structurally except
> for the placement of a single hydrogen.
> 
> Question 1:  is the level of theory we are operating at accurate enough to
> characterize these energy differences as being significant?
> 
> Question 2:  if these energy differences are meaningful (or if we can do
> them at a level that is more reliable), is there a way to extract out from
> the calculations why one tautomer is more stable than the other?
> 
> Thanks in advance,
> Joseph W. Bausch
> [email-masked]
> 
> 
> 
> 
> ---
> Administrivia: This message is automatically appended by the mail exploder:
> [email-masked]: Everybody | [email-masked]: Coordinator
> [email-masked]: HELP CHEMISTRY or HELP SEARCH | Gopher: www.ccl.net 73
> Anon. ftp: www.ccl.net   | [email-masked] -- archive search
>              Web: http://www.ccl.net/chemistry.html 
> ---
> 
>