• The real part is calculated using a Kramers-Kronig transformation of the imaginary part. If the imaginary part doesn't extend over a broad enough energy range, it can affect the accuracy of the transformation. Try expanding the energy range or increasing the number of k-points to see if this improves the real part.
• Convergence Parameters in SCF Calculation: Even though you’re using norm-conserving pseudopotentials, it’s important to ensure that parameters like k-point density, energy cutoff, and smearing (for metals) are well-converged. Small inaccuracies in the SCF calculation can have a larger effect on the real part of the dielectric function.
• You can follow the code of my papers given below to calculate the optical properties:

• Article Exploring the impact of strain on the electronic and optical...

• Article Analysis of the role of A-cations in lead-free A3SbI3 (A = B...

• Article Exploring new lead-free halide perovskites RbSnM3 (M = I, Br...

• The SCF code is different for calculating optical properties. Try to use the code in the data files at the end of these papers.
• And try to optimize the SCF file with proper k-point and cut-off energy

Md. Harun-Or-Rashid many thanks for your papers! I will examine. I have added these parameters in the SCF code as a following. Then, reel part is ok.

nosym =.TRUE.

noinv =.TRUE.

Merve Özcan You are also welcome.