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A basic level of a non-Boltzmann population distribution is available through setting the vibrational temperature, Tvib, which affects the relative populations of different vibronic states, or the spin temperature Tspin. For more complicated situations, such as simulating double resonance spectra or nascent population distributions following reactions there are two more general methods of specifying the population of levels. It is also possible to fit these populations to measured intensities or band contours, as the populations are simply parameters that can be floated as any other; see the sections on Line Intensity Fitting and Contour Fitting. It is also possible to simulate and fit time of flight or kinetic energy release spectra; use IntensityUnits = PopDist to ignore transition moments and just plot energy levels with populations derived as described in this section. Plots against velocity (in units of cm-1/2), rather than energy can be obtained by Setting PlotUnits to sqrtcm1.
Where many levels are involved, a general function instead of the simple Boltzmann equation can be used, as described under Custom Populations. Alternatively, the populations of each level can be specified individually as described below, and is most straightforward where only a few levels are involved.