Q1: The study addresses an ideal crystal without any defects. This is correct for the idealized case of zero temperature, T=0. Each real crystal however has inevitably equilibrium point defects for non-zero temperature and generally, non-equilibrium defects like dislocations. Hence, a question arises, how the presented results would change, if these important properties of real crystals were taken into account? I do not expect quantitative estimates, but just a qualitative answer – whether the observed decrease of the bandgap persists? Will it shift for larger or smaller strain? R1: To the best of my knowledge, the defects may introduce the “defect bands” (or intermediate bands) to the electronic bandstructure, which corresponds to the discrete eigenvalues of the Hamiltonian (in contrast to the continuous spectrum, which is related to the bands). Since the bandgap is defined as the difference between the conduction band minimum and the valence band maximum, technically, if these bands are not changed, then the bandgap value remains the same. However, the energy required to excite an electron to become a conduction electron decreases.
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