Perhaps you should try some spelling variations:
2 molecules » _ molecules, i molecules, 1 molecules
6 molecules » _ molecules, i molecules, ii molecules
2 clusters » 12 clusters, _ clusters, 6 clusters
2 cluster » 4 cluster, 3 cluster

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %22atoms%((2 molecules) OR (6 molecules))%((((2 clusters) OR (2 cluster))) OR (1((2 clusters) OR (2 cluster)))) AND plasmas%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22atoms%((2 molecules) or (6 molecules))%((((2 clusters) OR (2 cluster))) or (1((2 clusters) OR (2 cluster)))) and plasmas%22".
• Adding a wildcard symbol may retrieve word variants: %22atoms%((2 molecules) or (6 molecules))%((((2 clusters) OR (2 cluster))) or (1((2 clusters) OR (2 cluster)))) and plasmas%22*.