Knowledge Bank

Two procedures are presented for either neglecting or approximating the less important electron repulsion integrals that arise in ab initio calculations. Both techniques are charge conserving and have been employed in the recent development of a crystalline Hartree-Fock formalism. The first technique involves neglecting basis function products in the firstorder density matrix if the corresponding overlap integral is small.That is to say, all one-electron integrals as well as all electron repulsion integrals involving the product $fifj$ are neglected (set to zero) if . preliminary calculations on ethylene give the following results. [FIGURE] The second technique is designed to approximate electron repulsion integrals over contracted gaussian lobe functions.The approximation consists of replacing a many-term gaussian charge distribution with a single term which minimizes the least-square error in the description with a single term which minimizes the leas-square error in the description of the charge distribution. The following results were obtained for ethylene, approximating all elector repulsion integrals with magnitude less than T. [FIGURE] Results of these approximations on larger systems will be presented.