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From: ³Âº½Ñà (physics.hangyan.chen_at_gmail.com)
Date: Fri Apr 12 2013 - 15:40:20 CDT
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Hello All,
I would like to use paratool plugin for the analysis of force constant for
water molecule.
Firstly, I use Gaussian to do a frequency analysis. The input file was like
this:
%chk=input.chk
%nprocshared=8
%mem=1gb
#p MP2/6-31G** IOP(7/33=1) freq=intmodes
******
0 1
O 0.6735 0.97431 -0.821672
H 1.0774 0.25058 -1.30049
H 0.2246 1.4863 -1.49445
In the output file (.log), I found the the Hessian matrix was like this:
The second derivative matrix:
R1 R2 A1
R1 0.58069
R2 -0.01115 0.58051
A1 0.03420 0.03420 0.16835
As I think, 0.58069 and 0.59051 just represent the force constant for O-H
bonds in water. Considering the unit for each value in the above hessian
matrix is hartree*bohr^-2, the force constant for O-H bond should be about
0.58069*2240.9=1301.3 kcal/mol/A^2.
However, if I import the .log file using paratool in VMD, I found the force
constant for O-H bond is about 515 kcal/mol/A^2.
Why the result calculated from paratool is so different from the result
analyzed directly from hessian matrix?
I appreciate any help!
Thanks in advance!
Hang
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