Moreover, much of the work in the generative de novo design area has been focused on the development of maximally expressive methods whose purpose is to explore the entire chemical space. Our approach is different in this regard, since it specifically rewards the generation of molecules that are similar to known lead compounds. Another such method is the recently published MolDQN method, which maximizes a "drug-likeness" (QED) score while also maintaining similarity to the original molecule. Virtual screening is related approach to generative methods, and can also be a powerful method for finding hits and lead compounds with desired properties. However, a virtual screen is limited in regard to what is in the queried databases (i.e. it is not possible to find something that is not there).