COUPLED-CLUSTER ANHARMONIC-FORCE FIELDS, SPECTROSCOPIC CONSTANTS, ANDVIBRATIONAL ENERGIES OF ALF3 AND SIF3+

Anharmonic force fields and re-vibrational spectroscopic properties of AlF3 and SiF3+ have been investigated in detail, using the coupled cl uster method with single and double substitutions augmented by a pertu rbative treatment of triple excitations [CCSD(T)] with a triple zeta b asis set of 124 contracted Gaussian-type orbitals (cGTOs). A complete set of re-vibrational spectroscopic constants for each species has bee n calculated using second order perturbation theory. The geometry only was calculated with a quadruple zeta basis set (224 cGTOs). Our best estimates of the equilibrium bond distances r(e) (Al-F) and r(e) (Si-F ) are 1.624 Angstrom and 1.508 Angstrom, respectively, based on the qu adruple zeta CCSD(T) bond distances and corrections derived from spect roscopically related known molecules. The CCSD(T) fundamental frequenc ies are 689 cm(-1)(nu(1)), 301 cm(-1)(nu(2)), 951 cm(-1)(nu 3), and 24 1 cm(-1)(nu(4)) for AlF3 and 853 cm(-1)(nu(1)), 357 cm(-1)(nu(2)), 118 7 cm(-1)(nu(3)), and 307 cm(-1)(nu(4)) for SiF3+. Energies of a number of low lying vibrational states (less than or equal to 2400 cm(-1)) h ave been calculated by three methods: (1) standard second order pertur bation theory formulas; (2) Van Vleck perturbation theory based on rai sing and lowering operators to second order and to fourth order; and ( 3) a variational method. (C) 1997 American Institute of Physics.