Rings are calculated using MIB by molinspiration.com. We calculate Bemis-Murcko Scaffolds (argument: -scaffold). For each BM Scaffold, we enumerate ring systems (argument: -ringSystems). For each ring system, we enumerate simple rings (argument: -simpleRings). We have to fudge a bit on simple rings that break aromaticity, tautomers and so on. We hope that this ring-based view of purchasable chemical space will be useful notwithstanding a few problems. Like all of ZINC, you must agree to use it at your own risk.
NB as of March 13, 2012, only about 10% of the molecules in ZINC have been processed for rings. We expect to have 95% coverage by April 1, 2012.