CONTRIBUTION OF ELECTRON CORRELATION TO H-BONDS

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The importance of the H-bond in chemistry and biology has motivated a great deal of inquiry. Our current understanding of the fundamental nature of this bond rests in large part on ab initio partitioning of the various energetic components. However, the previous calculations have generally been limited to the SCF level and have thus omitted the effects of electron correlation. Recent work has provided convincing evidence that correlation plays a major role in H-bonding, contributing a large fraction of the binding energy and significantly influencing its radial and angular dependence. The current proposal is designed to systematically investigate the effects of electron correlation in H-bonding, which may be broken down into dispersion energy and the correlation correction to the first-order energy (part of intramolecular correlation). The former will be calculated directly via variation-perturbation theory and the Coulomb part of the latter through a multipole expansion. Their sum will be compared with supermolecule second-order many body perturbation theory (MP2) which contains both terms in addition to their exchange counterparts. Basis sets will be of double- quality, augmented with two sets of polarization functions, designed to maximize dispersion energy, minimize basis set superposition error, and accurately reproduce experimental multipole moments of subsystems. The data will be fit to pairwise expressions to produce a set of transferrable parameters, for purposes of supplementation of ab initio calculations of large systems which are limited to the SCF level and for incorporation into potentials for Monte Carlo and dynamics calculations.