POST-HARTREE-FOCK STUDY ON AR-HCO-HCO+ COMPLEXES - A CRITICAL-EXAMINATION OF EXPERIMENTAL-DATA( AND HE)

The second order Moller-Plesset (MP2) to the fourth order many-body pe rturbation theory including single, double, triple, and quadruple subs titutions (MP4(SDTQ)), coupled cluster with single and double excitati ons (CCSD), CCSD with perturbative triple excitations [CCSD(T)], quadr atic configuration interaction with single and double substitutions (Q CISD), and QCISD including noniterative triples contributions [QCISD(T )] ab initio correlated levels of theory have been employed in studies on molecular geometry and stability of the Ar-HCO+ and He-HCO+ comple xes. Triple-zeta split-valence (6-311G) and correlation consistent (cc -pVTZ) basis sets augmented with diffuse and polarization functions we re used. At applied levels of theory the predicted interaction energie s (corrected for the basis set superposition error and zero-point vibr ational energy) for Ar-HCO+ range from -3.21 kcal/mol (QCISD/6-311G(2d f,2pd) to -4.21 kcal/mol (MP4(SDTQ)/aug-cc-pVTZ), and for He-HCO+ vary from -0.12 kcal/mol (-42 cm(-1), MP2/cc-pVTZ) to -0.37 kcal/mol (-130 cm(-1), CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVTZ). The optimized Ar-H di stance (2.1392 Angstrom at QCISD/cc-pVTZ and 2.1222 Angstrom at MP2/au g-cc-pVTZ) agrees very well with the experimentally determined (2.13 A ngstrom) value, whereas when step-by-step higher level methods and bas is sets are applied, the predicted He-H distance (1.9156 Angstrom at Q CISD(T)/aug-cc-pVTZ dramatically retreats from the experimental value of 2.00 Angstrom. (C) 1996 American Institute of Physics.