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Actually, the source mentioned in my question does answer partially the question. Consider a monatomic gas with an electron cloud. Its response to an electric field is typically given in terms of an expansion. $P(t) = \varepsilon_0\Big[\chi^{(1)}E(t)+\chi^{(2)}E^2(t)+\chi^{(3)}E^3(t)+..\Big]$ When there is spherical symmetry, for a monatomic case, we can ...