Dear Zubaer, these differences (which are in fact not that large!) are numerical, and might come from whatever can be different in different supercells, namely i) mesh cutoff ii) k-points. Probably you thought already about both factors. However, > The max force on any atom in the supercells is < 0.0001 eV/Ang. is not any useful information, because this is simply a formal criterion when your relaxation stops. More interesting is, how do the forces behave when the atoms are moved off-center. Make usuall "eggbox effect" tests and you'll see. > Also I checked the k-mesh convergence in all the cases. And what does this mean, exactly? The crucial point is, again, - how do forces off-equilibrium depend on the k-mesh.
Best regards Andrei Postnikov > Dear SIESTA users, > I have a question regarding the convergence of FC calculations. I have > done > FC calculations with 2 atoms, 8 atoms and 64 atoms (for Gamma point > frequency) with > SuperCell_1 0 > SuperCell_2 0 > SuperCell_3 0 > The highest *optical frequencies* I get 520.0 in 2-atom, 517.0 for 8-atom, > and 513.0 for 64-atom cases. This is for c-Si. The max force on any atom > in > the supercells is < 0.0001 eV/Ang. Also I checked the k-mesh convergence > in > all the cases. Why the frequency is changing with the supercell size? > Also, I did 2-atom basis FC calculations with > SuperCell_1 0 > SuperCell_2 0 > SuperCell_3 0 > and > SuperCell_1 1 > SuperCell_2 1 > SuperCell_3 1 > First one produces 2-atom supercell, and 2nd one 54. Here, also I find > different answers. If I compare the *highest* values in the *.FC files > (which give* force constants* matrices) in these two calculations, I find: > 14.22 and 13.88, respectively. > Could anyone please suggest what's wrong or what things I would need to > check? > Any hint would be really helpful for me. Thanks in advance. > Zubaer
