The concept of "chemical universe," "chemical space," or "chemical compound space" is associated with a set of all possible molecules described by a multi-dimensional space that represents their functional and structural properties and the relationship of the molecules to each other . Although the chemical compound space would be valid for virtually any type of compound, it has been typically associated with small organic molecules. Practical applications of this concept include the study of epigenetic-focused compounds , covalent protein kinase inhibitors , human immunodeficiency virus (HIV) protease inhibitors , natural products, and new compounds resulting from combining fragments or scaffolds from itself (pseudo-natural products) . Also, there are applications of chemical space to food and flavor chemicals and metal-containing molecules .