Orbital relaxation and molecular properties of silicon compounds

A model that includes orbital relaxation in ab initio quantum chemical calc ulations of molecular systems is proposed. The role of orbital relaxation i n determination of molecular properties is studied irt the following series of silicon compounds: SiHnF4-n, (n = 0-4), H3SiX, and SiX4 (X = H, Cl, CH3 , OH, F). If is shown that orbital relaxation always decreases the total en ergy of a molecule and leads to better agreement between the calculated and experimental equilibrium distances and dissociation energies. The orbital relaxation effect decreases in the following series of chemical bonds: Si-F > Si-C > Si-O > Si-Cl > SiH. Calculation of the ionization energies of mole cules in terms of Koopmans' theorem including orbital relaxation gives bett er or worse agreement with experiment. inclusion of orbital relaxation does not improve the description of the dipole moments of polar molecules. Stud ying the contribution of orbital relaxation allows one to clarify the role of silicon d-orbitals in the electronic structure of silicon compounds. The silicon d-orbitals are shown to be of independent significance in the form ation of chemical Si-O and Si-F bonds.