28
structure. But, it turns out that the CDW structure of the TiS2 and the
symmetrical structure are very close in energy having the difference of the order
of 0.005 eV/atom and surprisingly these differences are captured with the above
procedure. On the other hand, the adsorption energy of the hydrogen is similar
on the symmetrical and the distorted structure in the case of CDW structures,
therefore, they have been categorized as symmetrical for consistency due to the
threshold of 0.01 eV/atom. Table 4.1 summarizes the results for the distorted
structures which are classified based on the space group (based on Herman-
Maugin notation) of the distorted structure and the size of the reduced unit
cell capturing the distortion. Symmetry analysis for the classification has
been performed using the tool given in Ref. 61. The cutoff of of 0.05 Å on
the rotations/translations has been used in order to allow for inaccuracies or
residual forces.
Table 4.1: Classification of different compounds exhibiting distortions based on
the space group (based on Herman-Maugin notation) of the distorted structure
and the size of the reduced unit cell capturing the distortions.
Class MX2 1
¯Group Unit cell Class MX2 Group Unit cell
2H CoS2 P1 2×2 2H CoSe2 P1 2×2
2H IrS2 P1 2×2 2H OsS2 P1 2×2
2H OsSe2 P1 2×2 2H PdS2 P1 2×2
2H PdSe2 P1 2×2 2H PdTe2 P1 2×2
2H ReO2 P1 2×2 2H ReS2 P1 2×2
2H ReSe2 P1 2×2 2H RhS2 P1 2×2
2H RhSe2 P1 2×2 2H RhTe2 P1 2×2
2H RuO2 P1 2×2 2H RuS2 P1 2×2
2H RuSe2 P1 2×2 2H ScS2 P1 2×2
2H ScSe2 P1 2×2
1T CoS2 P1 2×2 1T CrS2 P1 2×2
1T CrSe2 P1 2×2 1T FeS2 P1 2×2
1T IrS2 P1 2×2 1T IrSe2 P1 2×2
1T ReO2 P1 2×2 1T ReTe2 P1 2×2
1T RhS2 P1 2×2 1T RuS2 P1 2×2
1T RuTe2 P1 2×2 1T MoO2 P1 2×1
1T MoS2 P1 2×1 1T MoSe2 P1 2×1
1T MoTe2 P1 2×1 1T OsS2 P1 2×1
1T OsSe2 P1 2×1 1T OsTe2 P1 2×1
1T WS2 P1 2×1 1T WSe2 P1 2×1
1T WTe2 P1 2×1 1T ReS2 P2×2
1T ReSe2 P¯1
2×2 1T RuSe2 P¯1
2×2
1T TaO2 P¯1
2×2 1T CoSe2 P3m1 2×2
1T IrTe2 P3m1 2×2 1T NbO2 P3m1 2×2
1T OsO2 P3m1 2×2 1T RhSe2 P3m1 2×2
1T RuO2 P3m1 2×2 1T WO2 P3m1 2×2