open-quotes Neglect of diatomic differential overlapclose quotes in nonempirical quantum chemical orbital theories. II. A polynomial expansion for Δ-1/2 in terms of Legendre and Chebyshev polynomials

Several attempts to justify the Neglect of Diatomic Differential Overlap (NDDO) approximation in terms of a power series expansion for δ-1/2 (δ is the overlap matrix) have been made. The new approach for attaining an optimal approximation of δ-1/2 represented here transfers the problem of finding an optimal matrix polynomial to an approximation problem of real functions within the spectrum of δ. Best approximations are derived by use of Legendre and Chebyshev polynomials. 14 refs., 2 figs., 3 tabs