The neglect of diatomic differential overlap (NDDO) fragment self-consistent field method for the treatment of very large covalent systems

Abstract

We present a semiempirical NDDO procedure, called the fragment SCF (FSCF) method, to treat very large molecules. The covalent system is partitioned into a relatively small subsystem where substantial chemical changes take place and an environment that remains more-or-less unperturbed during the process. We expand the wavefunction on an atomic hybrid basis and perform an SCF procedure for the subsystem in the field of the iteratively determined electronic distribution of the environment. We wrote a program for the IBM ~lSC/560 computer and did several test calculations for a variety of large classical molecules. Protonation energies, proton transfer potential curves, rotational barriers, atomic net charges, and HOMO and LUMO energies, as computed by the exact version of the NDDO method, are fairly well reproduced by our approximation. Using the FSCF method, we calculated the molecular electrostatic potential on the van der Waals envelopes of the specificity pocket of trypsin and the lysine side chain of the bound substrate and visualised electrostatic complementarity. We developed a novel bulk phase Monte Carlo simulation technique and calculated the energy by the above approximation and applied the method to amorphous silicon (a&). Starting from a distorted tetrahedrally bonded random network model of a-Si with 2 16 atoms, we performed Monte Carlo simulations using the FSCF energy calculation. For the second and subsequent configurations, we exploited the feature of the Metropolis-Teller algorithm, namely, that, to generate a new configuration, we displace only a single atom. Thus the number of integrals to be calculated drastically decreases since only those have to be reevaluated that contain the coordinates of the displaced atom. After equilibration we obtained distribution functions being almost identical to the one corresponding to the distortion free tetrahedrally bonded network. The same technique was applied to liquid chlorosilanes. We found that Si-CI bonds elongate by 6 to 16 pm while H-Si-CI and C1-Si-CI angles change by 2-4" as compared to the gas phase.