The heart of any chemical registration system is the method used to determine whether or not a given compound is already registered. Rather than building chemical querying into the underlying database, lwreg handles duplicate identification by computing a hash for each molecule in the database. To check whether or not a candidate molecule has already been registered, lwreg calculates a hash for the molecule and then checks whether or not that hash is already in the database. This test is done using simple string comparisons and is very fast in any modern database system. The method for computing a registration hash is discussed below.