Definitions for "BSSE"
Basis-set superposition error. An insidious artifact traceable to the fact that one can seldom afford to use a really big basis set. It causes an extra decrease in energy (i.e. more negative energy, greater stability) when two systems (atoms or molecules) are brought together. The energy of one fragment is lowered because its orbitals can use the basis functions on the other fragment, even if the actual electrons and nuclei on the other fragment are not included in the calculation. With a complete basis set, there is no BSSE because the other fragment's basis functions are superfluous. BSSE is usually ignored in thermochemical calculations, except for studies of weak, non-bonded interactions. See " counterpoise."
(Basis Set Superposition Error) error introduced when the energy of two molecules modeled together is lower than the sum of the energies when modeled separately, because more basis sets are available for each fragment (more degrees of freedom) during the calculation. Minimal basis sets can have less BSSE because only diffuse functions can span a to b. A particular problem for binding energies of weakly bonded molecular complexes, less with more complete basis sets.