['Spectral', 'assignments', 'have', 'been', 'made', 'using', 'empirical', 'force', 'fields', '[', '15', ']', ',', 'Hartree', '–', 'Fock', 'calculations', '[', '10,16,17', ']', 'based', 'on', 'the', 'isolated', 'molecule', 'approximation', '.']