The trade-off was that we now had clusters that weren't the same size as their icosahedral counterparts. So, our optimization procedure involved removing the highest energy atoms until the fcc cluster was the same size as the icosahedral cluster. Our results show that the fcc lattice becomes the more favorable structure at about 5,000 atoms. This is in better agreement with experimental observation than earlier theoretical studies. However, we are still at least 2,000 atoms off of the experimentally-observed crossover point.
Some concession needs to be made for temperature.
Indeed, theoretical studies are done at a temperature of zero kelvins. Cluster experiments are generally performed at about 35 kelvins. Thermal excitation could be one factor in this discrepancy. Further insight is gained by studying the mechanisms behind the transition from icosahedral shells to a face-centered cubic lattice. For small clusters, a large portion of the atoms are on the surface, so the icosahedral structure is more favorable.