Geometric parameters and atom labelling used for the description of intermolecular interactions between two water molecules, a and b. (a) The distance between two oxygen atoms is dOO. Ha1 represents the hydrogen atom that has the shortest non-bonded O⋯H distance (Ob⋯Ha1), which is dOH. Hb1 represents the hydrogen atom that has a shorter Ha1⋯Hb distance, which is dHH. (b) The angle Oa—Ha1⋯Ob is α. Pa/Pb is the dihedral angle between two water molecule planes. The torsion angle Ha2—Oa—Ha1—Ob is THOHO. (c) Angles between vectors containing O—H bonds are denoted βn. β1 represents the angle Oa—Ha1⋯Ob—Hb1, β2 represents the angle Oa—Ha1⋯Ob—Hb2. Only β2 is shown, the others have been omitted for clarity.