Frequency-dependent dielectric function of semiconductors with application to physisorption

The dielectric function is one of the most important quantities that describes the electrical and optical properties of solids. Accurate modeling of the frequency-dependent dielectric function has great significance in the study of the long-range van der Waals (vdW) interaction for solids and adsorption. In this work we calculate the frequency-dependent dielectric functions of semiconductors and insulators using the GW method with and without exciton effects, as well as efficient semilocal density functional theory (DFT), and compare these calculations with a model frequency-dependent dielectric function. We find that for semiconductors with moderate band gaps, the model dielectric functions, GW values, and DFT calculations all agree well with each other. However, for insulators with strong exciton effects, the model dielectric functions have a better agreement with accurate GW values than the DFT calculations, particularly in high-frequency region. To understand this, we repeat the DFT calculations with scissors correction, by shifting the DFT Kohn-Sham energy levels to match the experimental band gap. We find that scissors correction only moderately improves the DFT dielectric function in the low-frequency region. Based on the dielectric functions calculated with different methods, we make a comparative study by applying these dielectric functions to calculate the vdW coefficients (C-3 and C-5) for adsorption of rare-gas atoms on a variety of surfaces. We find that the vdW coefficients obtained with the nearly free electron gas-based model dielectric function agree quite well with those obtained from the GW dielectric function, in particular for adsorption on semiconductors, leading to an overall error of less than 7% for C-3 and 5% for C-5. This demonstrates the reliability of the model dielectric function for the study of physisorption.