Complex sentence: Systems in which an energy barrier separates two regions of configuration space may suffer from poor sampling in Metropolis Monte Carlo runs, as the low probability of overcoming the potential barrier can leave inaccessible configurations poorly sampled--or even entirely unsampled--by the simulation.
Split into simpler sentences: Systems in which an energy barrier separates two regions of configuration space may suffer from poor sampling. In Metropolis Monte Carlo runs, the low probability of overcoming the potential barrier can leave inaccessible configurations poorly sampled--or even entirely unsampled--by the simulation.