New global optimisation method for classical intermolecular potentials

 

 

E. BOURASSEAU*, P. UNGERER*# and A. H. FUCHS*

*Laboratoire de Chimie Physique, UMR 8000 CNRS, Université de Paris Sud, bât. 349, 91405, Orsay Cedex, France

#Institut Français de Pétrole, 1-4 av. de Bois Préau, 92852 Rueil-Malmaison Cedex, France.

e-mail address: emeric.bourasseau@lcp.u-psud.fr

 

 

This study proposes a new global procedure to perform optimisations of semi-empirical intermolecular potential parameters on the basis of a large reference database. To obtain transferable parameters, Ungerer1 proposed an original method few years ago, based on the minimization of a dimensionless error criterion. This method allows the simultaneous optimisation of several parameters from a large set of reference data. However, the computational cost of such a method limits its application, because it implies the calculation of an important number of partial derivatives, calculated by finite differences between the results of several different simulations. In this work, we propose a new method to evaluate partial derivatives, in order to reduce the computing time and to obtain more consistent derivatives. This method is based on the analysis of statistical fluctuations during a single simulation.

 

To test the efficiency of this new method, we have optimised the Lennard Jones potential parameters of the unsaturated hydrocarbon groups using the anisotropic united atoms description, in order to predict equilibrium properties of olefins. We have performed the minimization of a dimensionless error criterion based on a reference set containing 30 data from 6 different olefins. This minimization implied the calculation of 120 partial derivatives obtained by only 12 different simulations. The resulting parameters are consistent with those previously determined for linear and branched alkanes. Test simulations have been performed at temperatures ranging from 150 to 510 K for several a-olefins (ethylene, propene, 1-butene, 1-pentene, 1-hexene, 1-octene), several b-olefins (trans-2-butene, cis-2-butene, trans-2-pentene), isobutene and butadiene. Equilibrium properties are well predicted, and critical properties can be evaluated with a good accuracy, despite the fact that most of the results constitute pure predictions. It is concluded that the AUA potential, due to a relevant physical meaning, can be transferred to a large range of olefins with good success. In addition, our results have been compared to those of Wick et al.2, Spyriouni et al.3, Nath et al.4 and Lisal et al.5.

 

(1) Ph. Ungerer et al., J. Chem. Phys. 112, 5499 (2000)

(2) C. Wick, M. Martin and J. Siepmann, J. Phys. Chem. B, 104, 8008 (2000).

(3) T. Spyriouni, I. Economou and D. Theodorou, J. Am. Chem. Soc., 121, 3407 (1999).

(4) S. Nath, B. Banaszak, J. De Pablo, J. Chem. Phys., 114, 3612 (2001).

(5) M. Lisal, W. Smith and I. Nezbeda, J. Phys. Chem. B, 103, 10496 (1999).