Additional graph-theoretic constraints may be of interest to further restrict the admissible class of graphs. One such property is planarity. Since almost all molecules have planar structural formulas, it may be of interest to restrict the search space in this manner to rule out molecules with too many geometric constraints. At least some of these can be captured at least approximately in terms of graph-theoretic properties, such as the number of small rings or excessively high connectivity. Many topological filter criteria have been proposed in an effort to generate a large database of potential organic molecules .