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The NCI collection is the chemically curated version of the most recent release of the Open Chemical Repository Collection of the National Cancer Institute (link: http://dtp.nci.nih.gov/docs/3d_database/Structural_information/structural_data.html). The NCI database contains very diverse molecules and has been widely used for virtual screening purposes. All entries have been processed by the mcule molecule registration system (link). Molecules with structural problems and inconsistencies have been filtered out.