er</code> calls the function to obtain the orientational distribution. The function must accept either one  (<code>beta</code>) or two input arguments (<code>beta</code> and <code>gamma</code>), in radians. The function must accept vector arguments and return a vector <code>P</code> containing probabilities for each orientation, that is <code>P(k)</code> is the probability of finding the spin centers with orientation (relative to the sample frame) specified by <code>beta(k)</code> and <code>gamma(k)</code>.