Anharmonic force fields from analytic CCSD(T) second derivatives: HOF and F2O

The recent implementation of analytic second derivatives for CCSD(T) (coupl ed cluster theory with single and double excitations augmented by a perturb ational treatment of connected triple excitations) has been combined with a numerical finite difference procedure to calculate cubic and semidiagonal quartic force fields. Computational details of this approach are outlined. Applications are reported for HOF and F2O. The CCSD(T) results are in excel lent agreement with experiment and represent a substantial improvement over the results obtained from MP2 (Moller- Plesset second-order perturbation t heory). (C) 1999 American Institute of Physics. [S0021-9606(99)30508-0].