Hi Prof. Julian,
I am using GULP in order to produce phonon dispersion curve for Single Layer Graphene(SLG). I am using two types of unit cells:
Case (a) SLG with 2 basis atoms with basis vectors at 60 degree to each other.
Case (b) SLG with 96 basis atoms with orthogonal basis vectors.
For Case (a) I am getting somewhat close dispersion and phonon density of states(PDOS) curves to what research articles have been producing (max. freq of ~50 THz).
But in Case (b) the dispersion curve has its maximum frequency increased by ~10 THz. Also the PDOS is slightly changed.
(1) Should there have such a change of frequencies. If yes then why? (Is it happening because of bigger dynamical matrix has now been created with 96 basis atoms?)
(2) A slight change in the x-direction basis vectors length without changing the coordinates of the atoms for Case (b) changes the dispersion curve. What should be an ideal basis vector in case of a rectangular unit cell. Should it be the maximum of x-coordinate of all the atoms of the structure?
I am also attaching the Input GULP file for your reference.
(I am using REBO potential with C-C bond length of 1.3968418 to produce a 60 X 4.19 Angs rectangular unit cell of SLG.)
disp.gin (5.2 KB)
Thanks and Regards.