Ab initio potential energy surfaces for He-Cl2, Ne-Cl2, and Ar-Cl2

The three-dimensional ground state potential energy surfaces for He-Cl-2, Ne-Cl-2, and Ar-Cl-2 have been calculated using the single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)]. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions. Single point calculations for approximate minima have also been performed with several other basis sets including the quadruple zeta basis set (aug-cc-pVQZ) with bond functions. For He-Cl-2 and Ar-Cl-2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne-Cl-2 the CCSD(T) approach predicts the T-shaped configuration to be lower in energy. The linear configuration has been found to be more sensitive than the T-shaped one to the changes of the Cl-Cl bond length with the interaction becoming weaker when the Cl-Cl bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. The interaction in the T-shaped configuration becomes slightly stronger for shorter Cl-Cl bonds. For He-Cl-2 and Ar-Cl-2 the larger zero-point vibrational energy of the linear configuration is responsible for making the T-shaped configuration the ground vibrational state. Vibrational effects further increase the difference in energy between the ground state T-shaped configuration of Ne-Cl-2 and its linear counterpart. (C) 1999 American Institute of Physics. [S0021-9606(99)30216-6].