  Optical measurements under externally applied stresses allow us to study the
materials' electronic structure by comparing the pressure evolution of optical
peaks obtained from experiments and theoretical calculations. We examine the
stress-induced changes in electronic structure for the thermodynamically stable
1T polytype of selected MX2 compounds (M=Hf, Zr, Sn; X=S, Se), using the
density functional theory. We demonstrate that considered 1T-MX2 materials are
semiconducting with indirect character of the band gap, irrespective to the
employed pressure as predicted using modified Becke-Johnson potential. We
determine energies of direct interband transitions between bands extrema and in
band-nesting regions close to Fermi level. Generally, the studied transitions
are optically active, exhibiting in-plane polarization of light. Finally, we
quantify their energy trends under external hydrostatic, uniaxial, and biaxial
stresses by determining the linear pressure coefficients. Generally, negative
pressure coefficients are obtained implying the narrowing of the band gap. The
semiconducting-to-metal transition are predicted under hydrostatic pressure. We
discuss these trends in terms of orbital composition of involved electronic
bands. In addition, we demonstrate that the measured pressure coefficients of
HfS2 and HfSe2 absorption edges are in perfect agreement with our predictions.
Comprehensive and easy-to-interpret tables containing the optical features are
provided to form the basis for assignation of optical peaks in future
measurements.
