Electric-dipole-coupled H₂O@C₆₀ dimer: Translation-rotation eigenstates from twelve-dimensional quantum calculations

We report on variational solutions to the twelve-dimensional (12D) Schrödinger equation appertaining to the translation-rotation (TR) eigenstates of H₂O@C₆₀ dimer, associated with the quantized "rattling" motions of the two encapsulated H₂O molecules. Both H₂O and C₆₀ moieties are treated as rigid and the cage-cage geometry is taken to be fixed. We consider the TR eigenstates of H₂O@C₆₀ monomers in the dimer to be coupled by the electric dipole-dipole interaction between water moieties and develop expressions for computing the matrix elements of that interaction in a dimer basis composed of products of monomer 6D TR eigenstates reported by us recently [P. M. Felker and Z. Bačić, J. Chem. Phys. 144, 201101 (2016)]. We use these expressions to compute TR Hamiltonian matrices of H₂O@C₆₀ dimer for two values of the water dipole moment and for various dimer geometries. 12D TR eigenstates of the dimer are then obtained by filter diagonalization. The results reveal two classes of eigenstates, distinguished by the leading order (first or second) at which dipole-dipole coupling contributes to them. The two types of eigenstates differ in the general magnitude of their dipole-induced energy shifts and in the dependence of those shifts on the value of the water dipole moment and on the distance between the H₂O@C₆₀ monomers. The dimer results are also found to be markedly insensitive to any change in the orientations of the C₆₀ cages. Finally, the results lend some support for the interpretation that electric dipole-dipole coupling is at least partially responsible for the apparent reduced-symmetry environment experienced by H₂O in the powder samples of H₂O@C₆₀ [K. S. K. Goh et al., Phys. Chem. Chem. Phys. 16, 21330 (2014)], but only if the water dipole is taken to have a magnitude close to that of free water. The methodology developed in the paper is transferable directly to the calculation of TR eigenstates of larger H₂O@C₆₀ assemblies, that will be required for more extensive modeling of crystalline H₂O@C₆₀.