Clusters containing open-shell molecules: Minimum-energy structures and low-lying isomers of ArnCH (X (2)Pi), n = 1 to 15

The size evolution of the equilibrium structures of open-shell ArnCH (X 2%) Van der Waals clusters is investigated for n = 1 to 15. We describe a meth od for combining pair potentials for Ar-CH and Ar-Ar interactions to obtain potential energy surfaces for ArnCH clusters. For each cluster size consid ered, the global and a few energetically close local minima are calculated using simulated annealing followed by a direct minimization scheme. Ar(2)Ch is found to have an unusually stable planar structure, which persists as a motif in larger ArnCH clusters and has a strong effect on their optimal ge ometries. The lowest-energy isomers of ArnCH with n = 3 to 11 have all Ar a toms in a shell around CH. The only exception is Ar4CH, where the fully sol vated isomer is 3 cm(-1) higher in energy than the optimal isomer with CH b ound to the surface of the Ar-4 tetrahedron. For n = 7 to 11, the minimum-e nergy structure of derives from the global minimum of the Ar n CH Arn+1 clu ster, by replacing the Ar atom at the bottom of the pentagonal bipyramid wi th CH. The lowest-energy structure of Ar12CH is that of the optimal icosahe dral Ar-13 cluster, with CH replacing one of the Ar atoms on the cluster su rface. This structure supports the proposition based on the spectroscopic d ata, that for clusters with Ar n CH about 10 to 50 Ar atoms CH resides on t he surface of Ar-n.