FNO - ANOTHER CHALLENGING MOLECULE FOR CORRELATION METHODS

Although FNO has long been considered a computationally challenging mo lecule, no thorough study of this species has been performed with meth ods of electron correlation beyond second-order Moller-Plesset perturb ation theory (MP2) and the singles and doubles configuration interacti on (CISD) method. We have investigated the equilibrium structure, vibr ational spectra, and relative energetics of FNO using high-levels of a b initio theory. Methods include third and fourth-order Moller-Plesset perturbation (MP3 and MP4), singles and doubles coupled-cluster (CCSD ) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The quadratic configuration interaction method including single, double, and triple excitations [QCISD(T)] method is also used. Computations using these methods are performed with medium and large basis sets. Geometries cal culated at the Hartree-Fock level are grossly in error, while going to the CISD, MP3, MP4, and CCSD methods produces increasingly better res ults. The CCSD(T) and QCISD(T) methods yield quite accurate structures ; surprisingly, the MP2 results are almost as good. The quality of the calculated relative energetics closely follows that of the structure predictions, except that the MP2 level of theory now performs slightly worse than MP3, MP4, and CCSD methods.