QUANTUM SIMILARITY MEASURES UNDER ATOMIC SHELL APPROXIMATION - FIRST-ORDER DENSITY FITTING USING ELEMENTARY JACOBI ROTATIONS

The elementary Jacobi rotations technique is proposed as a useful tool to obtain fitted electronic density functions expressed as linear com binations of atomic spherical shells, with the additional constraint t hat all coefficients are kept positive. Moreover, a Newton algorithm h as been implemented to optimize atomic shell exponents, minimizing the quadratic error integral function between nb initio and fitted electr onic density functions. Although the procedure is completely general, as an application example both techniques have been used to compute a 1S-type Gaussian basis for atoms H through Kr, fitted from a 3-21G bas is set. Subsequently, molecular electronic densities are modeled in a promolecular approximation, as a simple sum of parameterized atomic co ntributions. This simple molecular approximation has been employed to show, in practice, its usefulness to some computational examples in th e field of molecular quantum similarity measures. (C) 1997 John Wiley & Sons, Inc.