Global expression for representing diatomic potential-energy curves

We propose a three-parameter expression that gives an accurate fit to diatomic potential curves over the entire range of separation for charge transfers between 0<=δZ<=1. It is based on a generalization of the universal binding-energy relation [J. R. Smith et al., Phys. Rev. A 39, 514 (1989)] with a modification that describes the crossover from a partially ionic state to the neutral state at large separations. The expression is tested by comparison with first-principles calculations of the potential curves ranging from covalently bonded to ionically bonded. The expression is also used to calculate spectroscopic constants from a curve fit to the first-principles curves. A comparison is made with experimental values of the spectroscopic constants.