Anharmonic force field and vibrational frequencies of tetrafluoromethane (CF4) and tetrafluorosilane (SiF4)

Accurate quartic anharmonic force fields for CF4 and SiF4 have been calcula ted using the CCSD(T) method and basis sets of spdf quality. Based on the a b initio force field with a minor empirical adjustment, the vibrational ene rgy levels of these two molecules and their isotopomers are calculated by m eans of high order Canonical Van Vleck Perturbation Theory (CVPT) based on curvilinear coordinates. The calculated energies agree very well with the e xperimental data. The full quadratic force field of CF4 is further refined to the experimental data. The symmetrization of the Cartesian basis for arb itrary combination bands of T-d group molecules is discussed using the circ ular promotion operator for the doubly degenerate modes, together with tabu lated vector coupling coefficients. The extraction of the spectroscopic con stants from our second order transformed Hamiltonian in curvilinear coordin ates is discussed, and compared to a similar procedure in rectilinear coord inates. (C) 2000 American Institute of Physics. [S0021-9606(00)01403-3].