MOPAC ESP charges-problems.
Dear Netters,
I have a good experience in calculating atomic charges using the Electro-
statical Potential fitting (ESP). I've been using the MOPAC 6.0 ESP
program to calculate the point charges and then used them to
assign organic molecules for AMBER MM/MD calculations. So
far everything has been working successfully (the MM/MD calculations
have reproduced experimental data).
Now I have run into a problem:
Obviously some times the ESP fitting breaks down or fail to converge
to reasonable charges. I have calculated ESP charges of six different
conformations charged molecule (-1). The different conformations were
generated by a MD run and the minimized using MM. Three of these
conformations yield charges that are almost the same but the remaining
three conformations differs severely from each other and also gives
unreasonable magnitudes of the charges. Like +4 and -9 for the atoms in
a C=O group. What also can bee seen from the calculations are that the
RMS DEVIATION of the electrostatic potential from the ESP charges versus
the QM one is really large for these "failed" calculations.
I have not seen this problem be mentioned in any article about
any ESP programs (MOPAC ESP ot others).
Does anybody have an idea of the reason for this problem ?????
Could it be that a molecule in a packed conformation some
atoms are "hidden" under the ESP surfaces by other exposed atoms and
then the algorithm runs into troubles when calculating these hidden
atoms charges or is it just that the algorithm fails to converge
sometimes ?? or is it something else ????
I'll summarize the replies to the list !!!!
Greetings
/martin
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Martin Norin tel: +46-8-790 7512
Dept. Biochem fax: +46-8-723 1890
KTH e-mail: martin # - at - # physchem.kth.se
100 44 Stockholm
Sweden
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