MPEOE ("modified partial equalization
of orbital
electronegativity")
is a fast method for the determination of empirical net atomic charges
[1,2]. In this method, the net
atomic
charge on atom A is obtained by an iterative process. The
process
starts from an initial charge distribution, usually zero net atomic
charges
on all atoms, except on atoms in groups with non zero formal charge,
where
the sum of the net atomic charges should equal the formal charge. In
each
iteration, fractional charges are transferred between directly bonded
atoms
from the less electronegative to the more electronegative atom. The
amount
of charge transferred is equal to the difference of
electronegativities,
multiplied by an exponentially decaying bond type dependent damping
factor.
The electronegativities are assumed to be linear functions of the net
atomic
charge. The process attains convergence when the maximum amount of
charge
transferred along any bond is less than a given threshold. Using the
notation
![]() |
electronegativity of atom A in iteration n, |
![]() |
net atomic charge on atom A at end of iteration n, |
![]() |
charge transferred along bond AB in iteration n, |
![]() |
damping factor for charge transfer along bond AB, |
![]() |
factor in linear dependence of electonegativity on net charge, |
![]() |
electronegativity of the positive ion A+, |
the following formulae are used to compute net atomic charges in iteration n:
for each atom A, compute electronegativity: |
![]() |
for each bond AB, compute damping factor dn: | ![]() |
for each bond AB, compute charge transferred from A to B: | ![]() |
for each atom A, compute new net atomic charge: | ![]() |
The parameters ,
,
(atom parameters) and
(bond parameters) are stored in the Wit!P library file mpeoe.wlb,
from where they are read at program start-up. The parameters in this
file
were determined by a least squares parameter fitting procedure to
produce
net atomic charges which closely resemble charges from the MSI Quanta /
CHARMm 22 all-hydrogen parameter set. The hidden (i.e. not accessible
through
any of the menus) MPEOE command may
be used to derive alternative parameter sets.
Note: MPEOE charge calculations are always done for complete connected fragments.
1. K.T. No, J.A. Grant, and H.A. Scheraga:
Determination of net atomic charges using a modified partial
equalization
of orbital electronegativity method. 1. Application to neutral
molecules
as models for polypeptides.
J. Phys. Chem., 94:4732-4739, 1990.
2. K.T. No, J.A. Grant, M.S. Jhon, and H.A. Scheraga:
Determination of net atomic charges using a modified partial
equalization
of orbital electronegativity method. 2. Application to ionic and
aromatic
molecules as models for polypeptides.
J. Phys. Chem., 94:4740-4746, 1990.