Parametrization of the Scaling Factor in the Quasi-Relativistic INDO Method Based on the Equilibrium Geometries and Vibrational Frequencies of Electroneutral Closed-Shell XY4 Molecules Possessing the Td Symmetry

The parameter K in the quasi-relativistic INDO (QR-INDO) relation hiAjBcore = 1/2 K Si,j (βiA + βjB) was optimized by applying the QR-INDO method to the optimization of the geometries and calculation of vibrational wavenumbers of 36 molecules of the XY4 type with X = C, Si, Ti, Ge, Zr, Sn, Hf and Pb, and Y = H, F, Cl, Br and I. The calculations were performed for 10 values of K uniformly spaced across the region of 1.1 to 2.0. The optimum value with which the best fit of the bond lengths and vibrational frequencies was achieved was K = 1.5. This is very close to the value of 1.4 recommended in the literature. The study was performed using recent tabulated gas-phase data. The application of the variable scaling approximation approach was found generally unsuitable.