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I am performing ferroelectric calculations for Sb-doped Bi2Te3, a topological insulator with an energy gap of approximately 70 meV, using VASP.
should I use LDA or GGA?

Hi,

Thanks for reaching out. Both LDA and GGA are approximations that can sometimes yield physically inaccurate results, especially when it comes to the band gap. It's a priori quite difficult to say which approximation will give good results in a particular system. Therefore, I encourage you to experiment with LDA/GGA and maybe even some other flavors like PBEsol to find out what best describes your system and the physics you are interested in.

I will move this topic over to From Users to Users. Maybe someone else who already has experience with Bi2Te3 can contribute to this question.