We have implemented (starting with object generation 4.99) the Leng oscillator which has been developed to model optical constants of semiconductors. As shown in the original article (Thin Solid Films 313-314 (1988) 132-136) it works well for crystalline silicon:

Warning: While the model works fine in the vicinity of strong spectral features (critical points in the joint density of states) it may generate non-physical n and k values in regions of small absorption.