A method used to calculate the heat of dissociation of a molecule by extrapolation from observed band spectra. The dissociation energy D0 is equal to the sum of the vibrational quanta ΔG, where ΔG represents the energy between two successive vibrational states. This means that D0 is approximately equal to the area under the curve of ΔG plotted against the vibrational quantum number v. If the first few vibrational quanta are observed, an approximate value of D0 can be obtained by a linear extrapolation of this curve. If a sufficient number of vibrational quanta have been observed, a considerable improvement in the value of D0 can be obtained by taking the curvature of the ΔG curve into account by quadratic (and/or) higher terms. The technique was put forward by R. T. Birge and H. Sponer in 1926.