RKR1: A computer program implementing the first-order RKR method for determining diatomic molecule potential energy functions

•Efficient computer program implementing first-order RKR inversion procedure.•G(v) and B(v) functions may be represented represented by 3 different types of fx.•Internal convergence tests report the precision of the resulting turning points.•Can generate a PEC from only vibrational constants and an estimate of Re.•Can also automatically smooth over irregular or unphysical inner-wall behavior.

This paper describes computer program RKR1, which implements the first-order semiclassical Rydberg-Klein-Rees procedure for determining the potential energy function for a diatomic molecule from a knowledge of the dependence of the molecular vibrational energies Gv and inertial rotation constants Bv on the vibrational quantum number v. RKR1 allows the vibrational energies and rotational constants to be defined in terms of: (i) conventional Dunham polynomial expansions, (ii) near-dissociation expansions (NDE׳s), or (iii) the mixed Dunham/NDE “MXR” functions introduced by Tellinghuisen [J Chem Phys 2003; 118: 3532]. Internal convergence tests ascertain and report on the precision of the resulting turning points. For cases in which only vibrational data are available, RKR1 also allows an overall potential to be constructed by combining directly-calculated well widths with inner turning points generated from a Morse function. It can also automatically smooth over irregular or unphysical behavior of the steep inner wall of the potential.