Weak interactions of molecular hydrogen clusters: Novel 1D, 2D, 3D building blocks for ultracold H2 phases

Standard density functional theoretic and natural bond orbital analysis methods are employed to explore structural and energetic trends in the unexplored domain of ultraweak intermolecular interactions of H2. The structural motifs of optimized (H2)n clusters are found to differ appreciably from those of the familiar H-bond (or related X-ogen bond) interactions of polar molecules, representing a rich new realm of ultraweak binding topology and energetics. Specific examples include high-symmetry (Cartesian-like) 1D rods, 2D platelets, or 3D rhomboids of 4- or 6-coordinate monomers, strikingly different from the properties of disordered warm H2 vapors. We sketch some regularities and trends in this weak regime of chemical attraction and speculate on where such novel features of H2 aggregation may come into prominence.