Frequency dependent kernel

It is known that the exact Vignale Kohn kernel greatly improves the static polarizabilities of infinite polymers and nanotubes (see JCP 123 174910), but gives bad results for the optical spectra of semiconductors and metals. For the low frequency part one needs a frequency dependent kernel, because Drude-like tails are completely absent in the ALDA. With a modified Vignale Kohn kernel, neglecting μ_xc so that it reduces to the ALDA form in the static limit (see PRB 74 245117) much better results can be obtained. Band currently only supports the modified VK kernel in either the QV or CNT parametrization, and it should only be used for metals.