||Anharmonic force fields and re-vibrational spectroscopic properties of AlF3 and SiF3+ have been investigated in detail, using the coupled cluster method with single and double substitutions augmented by a perturbative treatment of triple excitations [CCSD(T)] with a triple zeta basis set of 124 contracted Gaussian-type orbitals (cGTOs). A complete set of re-vibrational spectroscopic constants for each species has been 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 quadruple zeta CCSD(T) bond distances and corrections derived from spectroscopically related known molecules. The CCSD(T) fundamental frequencies are 689 cm(-1)(nu(1)), 301 cm(-1)(nu(2)), 951 cm(-1)(nu 3), and 241 cm(-1)(nu(4)) for AlF3 and 853 cm(-1)(nu(1)), 357 cm(-1)(nu(2)), 1187 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)) have been calculated by three methods: (1) standard second order perturbation theory formulas; (2) Van Vleck perturbation theory based on raising and lowering operators to second order and to fourth order; and (3) a variational method. (C) 1997 American Institute of Physics.