Important Properties of Superposition Error

There are several salient facts to bear in mind  

In general, BSSE is reduced as the basis set becomes larger and more flexible. However, there is no strict correspondence, and the BSSE can in fact become larger with certain additions to the basis set. Minimal basis sets are particularly prone to large BSSE, as are those like 3-21G with a poor description of the inner shells. 

The BSSE rises rapidly as the two molecules approach one another angular dependence is more complex. 

The origin of BSSE makes it difficult to incorporate counterpoise corrections directly into gradients of the potential energy. 

Whereas SCF BSSE can be reduced to negligible proportions with large basis sets, the superposition error at correlated levels goes down much more slowly, persisting at large values, even with very flexible bases. 

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