From owner-chemistry@ccl.net Tue Oct  5 15:28:00 2010
From: "Kalju Kahn kalju+/-chem.ucsb.edu" <owner-chemistry]^[server.ccl.net>
To: CCL
Subject: CCL: AsMe3 Computing and correction enthalpy
Message-Id: <-42907-101005150225-13865-zAdjEACBTo0V+PcSlIc/vw]^[server.ccl.net>
X-Original-From: "Kalju Kahn" <kalju(-)chem.ucsb.edu>
Content-Transfer-Encoding: 8bit
Content-Type: text/plain;charset=iso-8859-1
Date: Tue, 5 Oct 2010 12:01:46 -0700
MIME-Version: 1.0


Sent to CCL by: "Kalju Kahn" [kalju/a\chem.ucsb.edu]
Osama,

Your proposed correction is not valid, in general entropy and enthalpy
are two different things and you should not expect that their errors
are (linearly) correlated.

Calculated errors in H and S may be correlated.  For example, a
computational method that underestimates the enthalpy of binding
of two monomers in a bimolecular complex will likely predict that
the the vibrational entropy due to the vibration along the complex
dissociation mode is too high (the complex is calculated to be too
loose).  But the relationship is complex enough that you can't use
the known enthalpy error to correct the entropy value.

Sometime, calculated errors in H and S have the same source.  An
example of this is the vibrational correction for H(thermal) and S.
It is well known that Hartree-Fock method overestimates harmonic
bond stretching frequencies by ca 10%.  Thus it is justified to
scale frequencies, and use scaled frequencies to calculate better
H(thermal) and S.  But in your case, it is the H(electronic) that
is wrong.

/sarcasm
  To get the correct Cp, multiply your calculated result by Tambov
  constant, to get the correct S, divide your number by Skinner constant
  if this fails develop your own density functional that will reproduce
  the experimentally observed values.
end sarcasm/


Kalju

>
> Sent to CCL by: "Osama T Akoubeh" [dolkeen__hotmail.com]
>
> The enthalpy result from chemkin is 2929.537 and from leterature 2800
> kcal/mol , is it possible chemically to correct the value by multiplying
> with a factor : The factor : 2800/2929.537 = 0.955782 to obtain a the
> corrected one and to multiply each thermodynamic properties such as Cp , S
> and H with this factor , so I can prepare a new Chemkin polynomial with
> corrcted one
>
> anyone can help to see if this procedure correct
>
> Osama

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Kalju Kahn
Department of Chemistry and Biochemistry
UC Santa Barbara, CA 93106