Structure and energetics of ArnNO- clusters from ab initio calculations

The potential energy surface for the ground state of the Ar-NO-((3)Sigma(-) ) complex has been calculated at the unrestricted Moller-Plesset perturbati on theory through the fourth order (UMP4). Calculations have been performed using the augmented correlation-consistent polarized triple zeta basis set supplemented with bond functions (aug-cc-pVTZ+bf ). The global minimum wit h a well depth of approximately 651 cm(-1) has been found for an approximat e T-shaped structure (R=6.28 a(o) and Theta=83.62 degrees). Two other, loca l minima correspond to two collinear forms, Ar-N-O and N-O-Ar. All these mi nima are located in the regions of local reduction of exchange repulsion. T he ground vibrational state dissociation energy D-0 determined by the collo cation method has been found to be 587 cm(-1). Several rovibrational states have been predicted and characterized. The ground-state complex assumes a fairly rigid T-shaped structure. The trimer, Ar2NO-((3)Sigma(-)), is also o f the T-shaped-cross structure, and the pairwise equilibrium and dissociati on energies amount to 1402 and 1260 cm(-1), respectively. The three-body no nadditive effects raise the equilibrium energy by 41 cm(-1). The minimum-en ergy structures of clusters Ar-n=2,Ar-3...,Ar-12-NO-((3)Sigma(-)) have also been found, from the pairwise additive potential, and ground-vibrational s tates have been simulated by means of rigid-body diffusion Monte Carlo. (C) 2000 American Institute of Physics. [S0021-9606(00)31323-X].