The characteristic property of the majority of molecular crystals is that the molecules in these solids retain their identity and that their intrinsic properties are modified only slightly by the intermolecular interaction. This is true in particular of the simplest molecular solids, composed of H2, D2, or HD molecules, at least at not too high pressures. The properties of these solids can be understood by treating the effect of the intermolecular interaction as a small perturbation on the properties of the free molecules. The analysis of the thermal, spectroscopic, and other properties of the solid hydrogens in terms of the properties of the free molecules and their interactions forms the main topic of this book. The free molecules are described to a high degree of accuracy by the Schrödinger equation for a system composed of two nuclei and two electrons moving under the influence of their mutual Coulomb interactions. In this first chapter we review the quantum theory of such systems, and we collect the available data on the free molecules to be used in the later chapters on the interacting molecules.
KeywordsHydrogen Molecule Ground Electronic State Adiabatic Approximation Multipole Moment Solid Hydrogen
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