Mixing enthalpies and Entropies



 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.
 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?
 Douglas E. Stack
 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