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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). |