ne identification of the global minimum energy configuration of molecular clusters has found many useful applications. including the calculation of solvent-solute interactions for phase equilibria prediction. Given the need to reliably identify the global minimum and other low-energy configurations, a fully deterministic global optimization algorithm is proposed. The potential energy is calculated using the OPLS force field [J. Am. Chem. Soc. 118 (1996) 11225] and the minimization problem is formulated using 3N-6 independent internal coordinates where N is the total number of atoms. A branch-and-bound framework is used to solve the problem. Tight convex underestimators have been derived for the non-convex terms and bounds for the dependent variables are calculated using interval arithmetic. The algorithm generates a converging sequence of upper and lower bounds. The algorithm is illustrated through the calculation of the global minimum configuration of a butane molecule, and a butane-ethylamine pair.