basis set superposition error

initialism: BSSE
https://doi.org/10.1351/goldbook.11409
An artifactual increase in calculated stability of the supersystem (the system formed by noncovalent interaction between two or more molecular entities, e.g. hydrogen bond system) resulting from the basis set of the supersystem being larger than for the component subsystems. The BSSE arises from a lowering of the quantum mechanical energy when the electron density of each subsystem spreads into the basis functions provided by the other subsystems.
Source:
PAC, 2016, 88, 239. (Glossary of terms used in computational drug design, part II (IUPAC Recommendations 2015)) on page 241 [Terms] [Paper]