Multilevel X-Pol as a Quantum Chemical Model for Macromolecules

The AExcd term can be determined by VMB expansion. For the bimolecular complex in Fig. 2.2, the two-body correction energy is exact. For condensed-phase and macromolecular systems, it is convenient to simply approximate AExcd by an empirical potential such as the Lennard-Jones potential or the Buckingham potential. 

The total binding energy between acetic acid and water were estimated to be -6.9 and -6.6 kcal/mol from M06/MG3S and CCSD(T)/MG3S, respectively. Therefore, Table 2.1 shows that the 

Note The 6-31G(d) basis set was used in all calculations with the M06/MG3S optimized monomer and dimer geometries. 

As explained elsewhere, exchange repulsion can be obtained as the difference between the energy from the antisymmetrized X-Pol wave function for the two fragments, and the X- 

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