# How to start with the optical properties of 2D materials with the VASP code?

I hope you are doing well. I am new to VASP, and my project is "Optical properties of 2D materials on VASP" but I have no idea how to run a simple calculation with VASP. I will be very thankful if someone guides me. Thank you so much.

• Are you comfortable with your understanding of DFT? (Here is a good and concise reference: tandfonline.com/doi/abs/10.1080/…) Nov 2 '20 at 11:53

The VASP manual is always the first resource to check, you can find it here. As a beginner, following "Getting Started" should help you greatly, from how to install the code, some details of the background theory, and how to run simple calculations.

Due to the enhanced coulomb interaction and the quantum confinement effect in the two-dimensional limit, the optical properties should be investigated with the GW+BSE method.

The practical calculation flow is the following if you are considering using VASP

• Perform a ground state DFT or hybrid calculation.
• Based on previous converged charge density and wavefunction, increasing the number of unoccupied orbitals to generate new charge density and wavefunction. (# of atoms*[100-200])
• Preform a GW calculation (keeping orbitals fixed) and calculate the quasiparticle energies and screened Coulomb kernel. (Keep the Wxxx.tmp and WFULLxxx.tmp files).
• Perform BSE calculation (the dielectric function will be written in vasprun.xml)

Related research paper on the optical properties of two-dimensional materials using GW+BSE implemented in VASP.

VASP's GW+BSE implementation

BSE calculation of silicon

• Hi @Jack Can you please explain a way to choose the value for NBANDS, OMEGAMAX (in step 2) and NBANDSO, NBANDSV (in step 4) for any systems? Any guess about the total time of the calculation in step -3 and 4 for a compound (number of atoms more than 5 may be)? I was working on TiN system and the job ran for around 6 days and ended up with an error (segmentation fault because of the exceeded wall time). Nov 28 '20 at 15:45
• 1. NBANDS is important for the response function in step3, the recommended setting is [atom number X 50--200], you should test the gap convergence to this parameter. 2. OMEGAMAX, NBANDSO, and NBANDSV in step4 decide how many states are contributed to your spectra, you can always start with NBANDSO=2 and NBANDSV=2.
– Jack
Nov 28 '20 at 23:48