FIG. 1.

Definition of covalent basis vectors for atoms in the water molecule. (a) For covalent dipoles centered at A (oxygen), the two basis vectors are ${\stackrel{\rightharpoonup }{e}}^{BA}$ and ${\stackrel{\rightharpoonup }{e}}^{CA}$, which are defined as unit vectors along its two O–H bonds whose two covalent dipoles are the same due to symmetry. (b) For covalent dipoles centered at C (hydrogen), the two basis vectors are ${\stackrel{\rightharpoonup }{e}}^{AC}$, the unit vector along the H–O bond, and ${\stackrel{\rightharpoonup }{e}}^{BC}$, the unit vector along the H–H virtual bond. The covalent dipoles centered at the other hydrogen atom B can be defined similarly.