BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS - IX - THE WEAKLY-BOUND COMPLEXES AR-H-2 AND AR-HCL

The interaction of Ar with H-2; and HCl has been studied using Moller- Plesset perturbation theory (MP2, MP3, MP4) and coupled-duster [CCSD, CCSD(T)] methods with augmented correlation consistent basis sets. Bas is sets as large as triply augmented quadruple zeta quality were used to investigate the convergence trends. Interaction energies were deter mined using the supermolecule approach with the counterpoise correctio n to account for basis set superposition error. Comparison with the av ailable empirical potentials finds excellent agreement for both bindin g energies and transition state. For Ar-H-2, the estimated complete ba sis set (CBS) limits for the binding energies of the two equivalent mi nima and the connecting transition state (TS) are, respectively, 55 an d 47 cm(-1) at the MP4 level and 54 and 46 cm(-1) at the CCSD(T) level , respectively [the XC(fit) empirical potential of Bissonnette et al. [J. Chem. Phys. 105, 2634 (1996)] yields 56.6 and 47.8 cm(-1) for H2 ( upsilon = 0)]. The estimated CBS limits for the binding energies of th e two minima and transition state of Ar-HCl are 185, 155, and 109 cm(- 1) at the MP4 level and 176, 147, and 105 cm(-1) at the CCSD(T) level, respectively [the H6(4,3,0) empirical potential of Hutson [J. Phys. C hem. 96, 4237 (1992)] yields 176.0, 148.3, and 103.3 cm(-1) for HC1 (u = 0)], Basis sets containing diffuse functions of (dfg) symmetries we re found to be essential for accurately modeling these two complexes, which are largely bound by dispersion and induction forces. Highly cor related wave functions were also required for accurate results. This w as found to be particularly true for ArHCl, where significant differen ces in calculated binding energies were observed between MP2, MP4, and CCSD(T). (C) 1998 American Institute of Physics.