Study of the structure of molecular complexes. IX. The Hartree–Fock energy surface for the H2O–Li–F complex

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Abstract
A large number of geometrical configurations (250) are computed with a large Gaussian basis set in the Hartree–Fock approximation for the H2O–Li–F complex. The many-dimensional potential energy surface has been sampled by keeping the molecule of water at a fixed position and by allowing the lithium and the fluorine to assume many positions in space. Because of the symmetry (C2v) of the water molecule, the 250 computations correspond to a sampling of about 600 configurations. The sampling includes a few highly repulsive configurations (up to about 300 kcal/mole in repulsion); the remaining points are either in the strongly attractive regions or in the weakly attractive regions of the surface. The stabilization energy of the complex reveals the existence of at least three possible structures: the Li–F–H2O structure (with C2v symmetry), with a stabilization energy (relative to H2O, F−, and Li+) of about −186 kcal/mole; a second Li–F–H2O structure with the fluorine forming a hydrogen bond (with one of the H–O groups of the water molecule), with a stabilization energy of about −191 kcal/mole; and a third structure, H2O–Li–F (with C2v symmetry), with a stabilization energy of about −201 kcal/mole. The main goal of this work is not the determination of the structures of the H2O–Li–F complex, but the construction of a reliable potential to be used in the study of the structure and properties of ionic solutions. For this reason, the computed Hartree–Fock energies have been accurately fitted with a simple analytical expression. In addition, the Hartree–Fock energy for the complex has been analyzed by partitioning it into two-body interaction energies (Li–F, Li+–H2O, and F−–H2O) and into three-body potential-energy terms. Finally, a second type of partitioning, the so-called bond energy analysis, has been presented and discussed.