We have suggested a procedure for evaluating the Pauli repulsion energy in hybrid quantum/classical (Q/C) treatments that is devoid of any system-specific parameterization and is completely ab initio [61]. Previous workers have modeled the purely quantum mechanical effect of Pauli repulsion by augmenting the mixed Q/C Hamiltonian with a van der Waals (vdW) term that prevents the electrostatic collapse of the solute and solvent molecules. The introduction of such a phenomenological term is problematic because """"""""atomic radii"""""""" [62, 63] are dependent on both molecular structure and electronic state, implying that the vdW term should be a function of these variables. We proposed to evaluate the Pauli repulsion energy by associating temporary (""""""""classical"""""""") wave functions with the classical region and then exploiting the equivalence of Pauli exclusion and permutational antisymmetry. The computational effort associated with the evaluation of the Pauli repulsion is negligible and therefore the method may readily be extended to large systems. The agreement between the proposed hybrid Q/C method and full quantum mechanical calculations [for the hydrogen-bonded water dimer and for MgOH+(H2O)n (n=1-4) clusters] suggests that the method may be applied to a wide variety of hydrogen-bonded systems.
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