Self-consistent-field (SCF) calculations have been used to determine the ground negative-ion and ground neutral potential-energy curves for HeH, ArH, and ArCl. In the systems studied, the X 1 potential of the negative-ion state is more repulsive at intermediate internuclear separations than the X2 potential of the neutral partners, leading to a curve crossing of the negative-ion state into the continuum. However, at small internuclear separations the negative-ion potential becomes less repulsive than the neutral potential, leading to a second curve crossing out of the continuum. The calculated potential-energy curves are used to provide estimates of the energy thresholds and cross sections for electron detachment in the collisions H- + He, H- + Ar, and Cl- - Ar. The angular thresholds for electron detachment in differential-scattering measurements are also given and compared to experimental data. © 1978 The American Physical Society.