# How can I find the binding energy for exciton (optical gap) with VASP?

What is the best way to find the binding energy for exciton using VASP? I have heard about the method of calculating it through the dielectric tensor with local field effects and effective masses, but I'm not sure. The computational resources do not allow me to perform the GW-BSE calculations. Thank you in advance for your help.

• To my knowledge, this is not accessible through Kohn-Sham DFT, unfortunately. GW-BSE is what is generally used. And also, the exciton binding energy is equal to the difference between the electronic and optical band gap. – Xivi76 Feb 5 at 18:03

$$\tag{1} E_{\mathrm{B}}=\frac{\mu}{\epsilon^2_{\infty}}R,$$
where $$\mu$$ is the reduced mass of the electron and hole, $$\epsilon_{\infty}$$ is the high frequency dielectric constant, and $$R$$ is the Rydberg constant. You can in principle construct all necessary parameters from a VASP calculation.