The potential energy surface for the ground state of the Ar–NO−(3Σ−) complex has been calculated at the unrestricted Møller–Plesset perturbation 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 with a well depth of approximately 651 cm−1 has been found for an approximate T-shaped structure (R = 6.28 ao and Θ = 83.62°). Two other, local minima correspond to two collinear forms, Ar–N–O and N–O–Ar. All these minima are located in the regions of local reduction of exchange repulsion. The ground vibrational state dissociation energy D0 determined by the collocation 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−(3Σ−), is also of the T-shaped-cross structure, and the pairwise equilibrium and dissociation energies amount to 1402 and 1260 cm−1, respectively. The three-body nonadditive effects raise the equilibrium energy by 41 cm−1. The minimum-energy structures of clusters Arn = 2,3…,12-NO−(3Σ−) have also been found, from the pairwise additive potential, and ground-vibrational states have been simulated by means of rigid-body diffusion Monte Carlo. © 2000 American Institute of Physics.