Click on the calculate button, or hit the space-bar, and wait briefly.
"debye" has now calculated a density of modes or density of states (DOS) for lead by randomly sampling q's in the BZ, and calculating the three mode frequencies for each q. The frequencies are assigned to frequency bins of a histogram, which is plotted at the right. Zoom in (right mouse button) on the peak at the lower frequency. It's evident that the bin width is too large to reveal any interesting details of the structure.
Change the number of bins from 30 to 200 and calculate. Now the resolution is better, but is all that fine structure real? Click on double MC steps (MC = Monte Carlo sampling of q-space), or hit the d-key (d for double) and watch what happens to the DOS curve. (DON'T click a second time until you see the change from the first click.) Each time you click, you double the number of MC steps, the number displayed in the right-hand graph giving the total number of q-points sampled. You get better data, but you also wait longer! (Warning: a RED display of the number of MC steps in the right-hand graph implies the graph is NOT appropriate to the parameters given in the panels. A new calculate is required!)
Compare the frequencies of the Van Hove singularities in the DOS curve on the right with maxima and minima of the calculated dispersion curves on the left.
Now click on the Debye toggle button to display the DOS used in the conventional Debye model for the heat capacity. The velocity vs slider may be changed to vary the only free parameter in the Debye model. (We assume the lattice constant to be known.) Choose log/log from the toggle above the graph to help get a good match to the low frequency portion of the DOS curve.