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396. ELPO:Electrostatic Isopotential Maps and
Interaction Energies from Localized Orbital
Contributions
by G. Naray-Szabo, CHINOIN Pharmaceutical and Chemical
Works, Budapest, Hungary
This program is based on the method outlined in Int. J.
Quant. Chem., 16, 265 (1979). The electrostatic
potential of a molecule in a given point, V(R), is
approximated by the following expression:
.
The integrals in the first (electronic) term are given
as:
ci2 = 0 for lone-
pair orbitals.
centres.
s-bond orbitals are constructed of atomic hybrids
directed along the bond:
( ia,s | R | ia,s ) = c2ia,2s ( ia,2s | R |
ia,2s ) + 2cia,2scia,2ps ( ia,2s | R | ia,2ps )
+ c2ia,2ps ( ia,2ps | R | ia,2ps ).
Lone-pair orbitals are oriented according to chemical
evidence, i.e., sp3, sp2 and sp hybrids are
tetrahedral, trigonal and linear, respectively.
Orbital coefficients can be taken as fixed parameters
or given in the input. p-orbitals are oriented
perpendicularly to the molecular or substituent plane.
Coefficients can be obtained from CNDO/2 calculations
on a suitable model system. All bond orbitals are
fixed to the molecule, and they are supposed to be
transferable between chemically similar systems. In
the second (nuclear repulsion) term of Eq. 1,
ZO(a,eff) is the effective nuclear charge of atom a.
Zeff = 1 for hydrogen and Zeff = Z-2 for atoms Li, ...,
F.
Integrals as given in an absolute coordinate system,
are calculated according to C. C. J. Roothaan, J. Chem.
Phys., 19, 1445 (1951).
The interaction energy of a system of point charges is
given as follows:
Here, Mc is the number of point charges, qa is the net
charge of a point (as calculated, e.g., by the CNDO/2
method) placed at Ra.
Several internal controls are included in the program
which help in producing input with self-explanatory
messages.
The computational work is proportional to the first
power of the number of bond orbitals. Estimated
computer time is 0.02 sec./orbital (CDC 3300) or 0.002
sec./orbital (UNIVAC 1108) for one point of the
isopotential map. Storage needed: 25,000 words.
Limitations:
600 atoms (including hypothetical lone-pair centres)
504 bonds (sigma + lone pair)
100 pi-bonds
Editor's Note:
This program was developed using hardware and software
which will be slightly unfamiliar to QCPE members. The
following fact must be kept in mind: The output from
the program as delivered by QCPE is on the line
printer. The printer that was used has a width of 160
columns as opposed to the more common 133-column
variety. Work will have to be done on the program to
convert it to CalComp output or something equivalent.
FORTRAN IV
Lines of Code: 650
Recommended Citation: G. Naray-Szabo, QCPE 13, 396
(1981).
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