Subject: Re: CCL:transition state

From: errol lewars <elewars*&*trentu.ca>
Date: Thu, 30 Jun 2005 12:54:30 -0400

2005 June 30

Hello,

Gaussian gives enthalpy and Gibbs free energy when you do a frequency 
calculation (which to be valid must be done using exactly the same level 
as was used for the geometry optimization, for example HF/3-21G for 
both). The values are printed right after the frequencies, e.g.

 Zero-point correction=                           0.173654 
(Hartree/Particle)
 Thermal correction to Energy=                    0.199937
 Thermal correction to Enthalpy=                  0.200882
 Thermal correction to Gibbs Free Energy=         0.117979
 Sum of electronic and zero-point Energies=          -2156.346839
 Sum of electronic and thermal Energies=             -2156.320555
 Sum of electronic and thermal Enthalpies=           -2156.319611
 Sum of electronic and thermal Free Energies=        -2156.402514

The values of interest to you are probably
(1) Sum of electronic and zero-point Energies,
(2) Sum of electronic and thermal Enthalpies,
(3) Sum of electronic and thermal Free Energies

(1) is the sum of the "raw" energy (electronic energy + internuclear 
repulsion, best located at the end of an optimization just before 
"RSMD", e.g.:
HF =-2156.520493|RMSD=1.429e-009...) and by subtraction from other 
(1)-type values gives 0 K enthalpy differences (since at 0 K S = 0).
(2) is the enthalpy at 298.15 K and 1.0000 atmosphere ("room temperature 
and pressure") . Subtraction from other (2)-type values gives enthalpy 
differences at room temperature/pressure.
(3) is analogous to (2); substitute "Gibbs free energy" for "enthalpy".

Look at the relevant Gaussian White Paper on their website.

As for the Hessian of a transition state, I suppose you want to see if 
this has one negative eigenvalue; so just ask Gaussian to calculate the 
frequencies (I assume you have the optimized TS; that's another problem) 
and see if there is exactly one imaginary vibrational freq.

EL
=====

QnUqDH wrote:

>Dear sir:
>  I have two questions to ask you. first,how can we get enthalpy and Gibbs
>free energy from the output file of frequence calculation. sencond,   how to
>caculate Hessian in finding transition state in Gaussian 98.
>
>  
>
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