A method used to calculate the heat of dissociation of a molecule by extrapolation from observed band spectra. The dissociation energy D 0 is equal to the sum of the vibrational quanta ΔG, where ΔG represents the energy between two successive vibrational states. This means that D 0 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 D 0 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 D 0 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.