Exploring the unknown chemical space in a meaningful way has always been one of the major objectives in drug discovery. Given the fact that the drug-like chemical space is enormous (the lower estimation is 10 23 molecules) , it cannot be easily searched. One of the most interesting attempts to understand the chemical space is the GDB project , which encompasses a set of databases that combinatorially enumerate large parts of the small molecule fragment-like chemical space. Currently there are databases that enumerate most fragment-like molecules with up to 13 (975 million molecules) and 17 (166 billion molecules) heavy atoms. Another approach, GDB4c , enumerates ring systems up to four rings both in 2D (circa one million ring systems) and 3D (more than 6 million structures).