Modelling nanostructured materials with force fields including fluctuating

charges

 

F. Calvo

 

IRSAMC, Universite Paul Sabatier

118 Route de Narbonne

F31062 Toulouse Cedex, France

florent@irsamc.ups-tlse.fr

 

 

 

At the statistical level, numerical modelling of complex molecular systems

requires some simplifications for efficiency purposes. Global optimization and

finite temperature simulation over long time scales (or on large samples)

are often beyond the present possibilities of first-principles approaches.

In this talk, we present some applications of empirical force fields with

fluctuating charges to nanosize objects, which exhibit heterogeneous bonding.

The fluc-q model allows one to incorporate explicitely the effects of charge

transfer. In the first example, the interaction of N alkali atoms on a

fullerene reveals a structural transition from homogeneous coating at low N,

to droplet formation at about N=8. Our second example deals with magnesium

oxyde clusters, which are known to exhibit a transition from covalent/ionic

to pure ionic bonding as the size grows to the crystal limit. By including

the effects of atomic polarization in a self-consistent way, we show that

the crossover between these two types of bonding occurs at several hundreds

of molecules.