see attachment：
the author adopted a rectangular cell (contains six carbon atoms) for stress- strian calculation along zigzag and armchair directions. However, a rhombic unit cell (contains two carbon atoms) was used for phonon calculation to describe the system instability under large deformation.
My get confused that a series of strians were applied on a rectangular cell ,how can a phonon calculation under large deformation adopted a rhombic unit cell?
can anyone help me ? or my comprehension is wrong ?
the reference is from 《Ab initio calculation of ideal strength and phonon instability of graphene under tension》.
For a hexagonal system, you can get the rectangular cell by longer lattice vectors from the primitive unit cell(from a1,a2 to 2a1-a2,a2). The reason of why the author chose the rectangular to do the stress- strian calculation is that it is more convenient and reliable to apply the uniaxial strain. After you applied uniaxial strain on rectangular cell and relaxed, you can get the strained primitive unit cell. And then for the phonon calculation. Although strained primitive cell is a rhombic unit cell, the value of angle is not 60. For AC tension strain, it's smaller than 60; opposite for the ZZ direction.
Thanks for your reply sincerely.
"After you applied uniaxial strain on rectangular cell and relaxed, you can get the strained primitive unit cell. " you mean transform the deformed rectangular cell into the strained primitive unit cell just as " get the rectangular cell by longer lattice vectors from the primitive unit cell(from a1,a2 to 2a1-a2,a2)" by redefining the lattice vector?
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.72.115411
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