Dimerization energies and basis set superposition error

5 kJ mol and this is an intrinsic barrier to the development of chemical theories of intermolecular bonding. 

The energies of the two separate fragments are subtracted fi om the total energy of the dimer. A key problem for the success of such a calculation is the choice of the basis set. A dimer calculation done by a method that does not go beyond the HF limit will only represent those energy contributions that do not stem from electron correlation, that is, mainly the purely electrostatic contributions. In fact, the difference between the energy calculated by a correlated wavefunction and those obtained by a HF wavefunction is often taken as a measure of the dispersion energy contribution. 

The application of equation 3.62 suffers from another technical difficulty, called basis set superposition error (BSSE). For the calculation of the total energy of the dimer, Fab. obviously the basis set is the sum of the basis sets of the separate fragments. In this way, each fragment in the dimer is allowed to use the external parts of 

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