The UBI-QEP method: A practical theoretical approach to understanding chemistry on transition metal surfaces

In this review we examine the presently available theoretical techniques for determining metal surface reaction energetics. The unity bond index-quadratic exponential potential (UBI-QEP) method, which provides heats of adsorption and reaction activation barriers with a typical accuracy of 1–3 kcal/mol, emerges as the method with the widest applicability for complex and practically important reaction systems. We discuss in detail the theoretical foundations of the analytic UBI-QEP method which employs the most general two-body interaction potentials. The potential variable, named a bond index, is a general exponential function of the two-center bond distance. The bond indices of interacting bonds are assumed to be conserved at unity (up to the dissociation point), and we cite state-of-the-art ab initio calculations to support this assumption. The UBI-QEP method allows one to calculate the reaction energetics in a straightforward variational way. We summarize the analytic formulas for adsorbate binding energies in various coordination modes and for intrinsic and diffusion activation barriers. We also describe a computer program which makes UBI-QEP calculations fully automated. The normalized bond index-molecular dinamics, (NBI-MD) simulation technique, which is an adaptation of the UBI-QEP reactive potential functions to molecular dynamics, is described. Detailed summaries of applications are given which include the Fischer-Tropsch synthesis, oxygen assisted X-H bond cleavage, hydrogen peroxide, methanol and ammonia syntheses, decomposition and reduction of NO, and SOx chemistry.