Molecular Simulation/Calculation of molecular forces using quantum chemistry

Intermolecular Interactions
Intermolecular interactions can be calculated using quantum chemical programs.
 * $$\Delta E_{interaction}=E_{complex}-\sum_i E_{fragment,i}$$

The fragments are the atoms or molecules that comprise the complex.

Basis Set Superposition Error
Many quantum chemical programs use atom centered basis functions to represent the atomic orbitals. For a given calculation, the number of basis functions used is fixed and finite. Inevitably, the description of the orbitals will be incomplete, in that a more accurate answer could be obtained if more basis functions were added.

When calculating intermolecular interactions, the calculation of the $$\Delta E_{interaction}$$ will have the basis functions from all the fragments. At close intermolecular contacts, the tails of these orbitals will extend close to the space of atoms in other fragments. In this way, the fragments will be stabilized by the presence of these additional basis functions, so the energy of the complex includes both the stabilization due to the intermolecular interaction and this "basis set superposition error."