Subject: thermochemistry?

From: ARTHUR HOAG <[email-masked]>
Date: Mon, 14 Feb 1994 16:24:16 -0500

I saw someones post regarding thermodynamic output from gaussian
and have a couple questions my self concerning  both gaussian and spartan.

I wish to calculate delta G and delta G* for a reaction.
I have already done the ab initio optimization and frequency calculation
thus I have E in hartrees (I converted to kcal/mol as well), enthalpy at 
298.15 K and 1 atm for vibration, translation, and rotation. I also have
entropy at STP for vibration, rotation, and translation (I dont have a
calculated electronic portion)

To calculate the G's so I can calculate delta G's I use
		G=H-TS
but, I need to know a couple things about H and S

1. Is H equal to E + Hvib + Hrot + Htrans? or do I need to add a pressure
and volume correction? (the reaction is a fragmentation (loss of fluorine))
2.  Do I add the H's to a zero point corrected E or not? I assume uncorrected 
because Hvib should take care of that?
3. Is S=Svib + Srot + Strans?  Sel is small?  Do I also need to subtract 
at term such as nR(ln(nN)-1).  I have tried this using my spartan output
Svib +Srot +Strans but keep getting a negative absolute entropy which
obiously doesnt make sense (note I am not talking about and entropy change).

I am also interested in learning the common conventions for this sort
of information.  I have consulted my spartan and gaussian user guides
with no luck.  I have also consulted "ab initio molecular orbital theory"
by Hehre, Radom, Schleyer, and Pople. As well as Exploring Chemistry with
Electronic Structure Methods: A guide to using gaussian with no luck.  If
there are better sources for this info I would appreciate suggestions 
(excluding typical Statistical Thermodynamics books because these have
not been clear to me. for example one included the nR(ln(nN-1) term while
another did not.

I think that is about all for now.  Please send responses to 
[email-masked] (and let me know if it is ok to direct
further questions to you personally in response to your response)

Arthur Hoag
[email-masked]
ph 814-863-7980