Observations on Calculations of Configuration Interaction and Basis Set Superposition Error

In this paper a novel way of presenting CI theory is reported that offers many benefits to students when compared to traditional methods. Here, CI is considered from the point of view of space expansion by the products of one-electron orbitals which originate from orbital mixing. This approach, which is complementary to mathematical derivations, provides a smooth transition for students to understand CI from their knowledge of HF. The basis set superposition error (BSSE) can also be considered in a similar way.