Methane octopole moment

The three moments higher than the quadrupole are the octopole, hexapole and decapoli. Methane is an example of a molecule whose lowest non-zero multipole moment is the octopole. The entire set of electric moments is required to completely and exactly describe the distribution of charge in a molecule. However, the series expansion is often truncated after the dipole or quadrupole as these are often the most significant. 

This is seen from a calculation of the electric moments. Methane has no nonzero dipole or quadrupole moments, but the xyz component of the octopole is 1.144 D. ... 

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