The present study describes the characterization of energy and structure of HBr21 in its low-lying electronic states, achieved through an extension of a new empirical method @Chem. Phys. Lett. 379, 139 ~2003!# recently introduced to evaluate the interatomic interaction in the HX21 (X 5F,Cl,Br,I) molecular dications. The method is based on identification of the main components of the interaction and their evaluation through some simple correlation formulas. Potential energy curves, given in a simple, natural, and analytical form, made possible the calculations of some important properties, such as double-photoionization energy thresholds, vibrational spacing, average lifetime, and Franck–Condon factors. The predictions, compared with data available in the literature, are of great interest for the analysis and interpretation of some new experimental results.