Noncovalently Bound Molecular Complexes Beyond Diatom-Diatom Systems: Full-Dimensional, Fully Coupled Quantum Calculations of Rovibrational States.

The methodological advances made in recent years have significantly extended the range and dimensionality of noncovalently bound, hydrogen-bonded and van der Waals, molecular complexes for which full-dimensional and fully coupled quantum calculations of their rovibrational states are feasible. They exploit the unexpected implication that the weak coupling between the inter- and intramolecular rovibrational degrees of freedom (DOFs) of the complexes has for the ease of computing the high-energy eigenstates of the latter. This is done very effectively by using contracted eigenstate bases to cover both intra- and intermolecular DOFs. As a result, it is now possible to calculate rigorously all intramolecular rovibrational fundamentals, together with the low-lying intermolecular rovibrational states, of complexes involving two small molecules beyond diatomics, binary polyatomic molecule-large (rigid) molecule complexes, and endohedral complexes of light polyatomic molecules confined inside (rigid) fullerene cages. In this Perspective these advances are reviewed in considerable depth. The progress made thanks to them is illustrated by a number of representative applications. Whenever possible, direct comparison is made with the available infrared, far-infrared, and microwave spectroscopic data.