In physics, dispersion is caused because different wavelengths of light (colors) have different speeds than each other depending on the medium, and therefore have more or less refraction than other wavelengths.
Now in Yafaray, you have the basic IOR field, as seen below. But, the dispersion field is simply one field. To get accurate dispersion, you would have to enter the IOR for every single wavelength of light, because dispersion is not linear. It is a complicated relationship. But, since Yafaray only has a single slider (Dispersion Power) that affects dispersion, I'm not sure if it is the right tool.
Here is a good list of refractive indices.
My comment stated: 'Two words use Cycles.', but I didn't know that Cycles doesn't have dispersion/caustic support (YET). You might want to check out LuxRender, but I believe that it hasn't been updated since 2009.