Leng model for optical constants

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.