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From: "Douglas E. Stack" <destack ":at:" unomaha.edu>
To: "'CCL'" <CHEMISTRY ":at:" www.ccl.net>, "'spartan'" <spartan ":at:" wavefun.com>
Subject: Mixing enthalpies and Entropies
Date: Tue, 3 Feb 1998 12:37:42 -0600
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Dear CCLers:
I would appreciate input for the following thoughts.  I have molecules =
who's size prohibits geometry Opt and freq. calculations at the HF/6-31G =
level.  I've been getting geometries  with PM3 semi-empirical, then =
submitting the geometry to a single-point HF6-31G calculation for energy =
data (enthaphy only, can't do freq. calc. on the PM3 geometry).  For =
some of these reactions (addition reactions A+B-->C) entropy changes =
could be important so I'd like calculate deltG of the reaction not just =
deltaH. =20
My question is, can I use energy data from the HF6-31G calculation for =
my enthaphy terms (deltaH), and mix them with my entropy data obtain =
from PM3 semi-empirical  frequency calculations. Will the small =
contributions from entropy matter? And because there smaller than =
enthalpy, is there more room for error in the entropy terms?=20


Douglas E. Stack=09
Assistant Professor
Department of Chemistry
University of Nebraska at Omaha
Omaha, NE 68182-0109
(402) 554-3647
(402) 544-3888 (fax)
destack*- at -*unomaha.edu