Force field parameters for Sn ?



 Dear colleagues,
 I need to carry out molecular mechanics calculations on organic
 molecules containing Sn atoms, more specifically, to investigate
 equilibrium structures of those molecules and clusters.
 In order to select a suitable force field, I'd like to know what are the
 parameters for Sn (actually, involved in a -(Sn)H2-alkyl substituant on
 pyridines) available so far, and for which force fields.
 I consider the UFF force field, but I'm not sure it is the most suitable
 ?
 Thanks for any advice.
 Regards
 --
 Didier MATHIEU
 CEA - Le Ripault, BP 16
 37260 Monts (France)
 Tel. 33(0)2.47.34.41.85