From: º (physics.hangyan.chen_at_gmail.com)
Date: Fri Apr 12 2013 - 15:40:20 CDT

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