IsoSpec is a fine structure calculator used for obtaining the most
probable masses of a chemical compound given the frequencies of the composing
isotopes and their masses. It finds the smallest set of isotopologues with
a given probability. The probability is assumed to be that of the product of
multinomial distributions, each corresponding to one particular element and
parametrized by the frequencies of finding these elements in nature. These
numbers are supplied by IUPAC - the International Union of Pure and Applied
Chemistry. See: Lacki, Valkenborg, Startek (2020)