New approaches to the description of short-range repulsion interactions inhybrid quantum/classical systems

We discuss some limitations of combined quantum mechanics and molecular mec hanics methods, which are becoming popular when working with large chemical and biochemical systems. More precisely, we focus on the treatment of the short-range interaction between a quantum molecule and a classical system. Generally, this short-range energy is handled by a sum over atom pairs of A /R-12 terms, where R is the atom-atom separation, as in Lennard-Jones poten tials. We illustrate the limitations of this approach and propose two other methods. The first one consists in the use of a Buckingham-type formula wh ich, compared to the A/R-12 term, is mathematically more flexible and physi cally more adequate. The second one is based on the use of a pseudopotentia l for describing the classical atoms that interact with the electronic clou d of the quantum system. The transferability of these potentials is discuss ed. (C) 2000 Elsevier Science B.V. All rights reserved.