Reference:

J. Chem. Phys. 40, 3596–3598 (1964) https://doi.org/10.1063/1.1725058

T. J. Swissler and R. Rein, “Calculations of molecular octopole moments for water and methane,” Chem. Phys. Lett., vol. 15, no. 4, pp. 617–620, Sep. 1972, doi: 10.1016/0009-2614(72)80388-9.