I am trying to use openbabel (version 2.4.1) to calculate approximate charges
using qtpie and qeq. Unfortunately, the results that I obtain appear completely
unreasonable. For example, using the following .xyz file for water (O-H = 0.947
A, HOH = 105.5):
water.xyz
----------
3
water
O 0.0000 0.0000 0.0000
H 0.7538 0.5732 0.0000
H -0.7538 0.5732 0.0000
----------
I obtain a qtpie charge of -1.5083 and a qeq charge of +1.8296 on the oxygen
atom. Other charge models give values much more in line with expectations
(eem(qO) = -0.6501, gasteiger(qO) = -0.4105, and mmff94(qO) = -0.8600). I used
the following command to determine these charges:
obabel water.xyz -omol2 --partialcharge qtpie
Thinking that this might be due to the fact that the water molecule is not
modeled well by these approximate methods, I looked at phenol (this molecule
was included in the original qtpie paper (Chen, Martinez, J. Chem. Phys. Lett.,
2007, 315)). Again, unreasonable charges were obtained the qeq and qtpie
charges were obtained. For example, using qtpie the calculated charge on the O
was -1.4532 and the charge on the aromatic carbon bound to the OH group was
+2.0941. I obtained the same results using the precompiled version of openbabel
included with Linux Mint (version 2.3.2) on a different computer.
It should be noted that the source code for the qtpie routine indicates that
some changes were made from the published procedure, so the values obtained
from openbabel are not expected to be identical. However, this note also
indicated that very similar values should be obtained.
Am I the only person having this problem? I would like to know if there is
something I am missing or is there a error in the implementation of the qeq and
qtpie charge models?
Thank you.
Clarke
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