The inclusion of exchange repulsion terms in the explicit polarization (X-Pol) model is examined by antisymmetrizing the X-Pol Hartree product wave function; this yields X-Pol with full eXchange, called X-Pol-X. When the monomers are treated by Hartree-Fock theory, this calculation can be accomplished by using the formalism of block-localized wave functions (BLW) that has been used in a variety of applications. In this case the block-localized structure in the X-Pol-X wave function allows for decomposition of the full Fock matrix of a dimension of M blocks into M smaller Fock matrices. The method is illustrated by considering two trimer structures of water clusters, and it is found that the total exchange repulsion energies in these hydrogen-bonding test cases are adequately treated andsto a good approximationsare pairwise additive. We also present a formalism to yield a simplified Fock matrix by making use of the neglect of interfragment differential overlap (NIDO) approximation, which is less severe than the neglect of diatomic differential overlap (NDDO) approximation.
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Acknowledgment. We are grateful to Peng Zhang, Hannah R. Leverentz, and Carlos Sosa for valuable discussions. This work has been partially supported by the National Institutes of Health (Grant GM46736 for the development of effective Hamiltonian methods for enzymatic processes and Grant RC1-GM091445 for the development of a quantal force field for biomolecular simulations), by the National Science Foundation (Grant CHE09-57162 for the development of mixed molecular orbital and valence bond theory and Grant CHE09-56776 for incorporating quantum mechanics in the treatment of complex reactive dynamical systems).