Quantitative Structure-Activity Relationships (QSAR) are empirical relationships that use molecular descriptors to quantify a specific biological activity or chemical property from the molecular structure. Typically, QSAR is used to refer to a process in which the structures of a set of compounds are quantified and then trained against their numerical values of the biological activity or physical property. The result is a mathematical model that can be used to predict the activity or property value of new compounds.
The independent variables in the QSAR equation are given in terms of the molecular descriptors, or operators on the molecular graph that strive to characterize the molecular structure. Such descriptors are based on molecular orbital theory, molecular topology, and molecular properties (lipophilicity, electronic, thermodynamic, quantum-chemical).
The quality and predictive capacity of the QSAR equation depends on the size and the diversity of the molecules found in the training set. The larger and more diverse the training set, the better suited the QSAR equation is equipped to predict activities of a new compound.