Dear Balabi,
great that you did these tests - it's exactly the way to operate if one wants to understand how things work. > We can see that except for > ecutrho=75, dispersion for all other cases are > quite close. The biggest > deviation for ecutrho=100 to 450 is 8.2275cm^-1, while for ecutrho=150 > to 450, the biggest deviation is almost as negligible as 0.7cm^-1. This > is quite surprisingļ¼because the tutorial link recommended high > ecutrho=450 for converged calculation. Well, not sure if the tutorial referred to the same pseudopotential, but you should indeed trust your calculations. > > Besides, I extract X point frequency from the above dispersions and > plot them with respect to ecutrho ( here is the image > https://pasteboard.co/GDEqdDb.png ) > On the other hand, I can also calculate phonon frequency at X point > (0,1,0) using ph.x directly with the same scf parameter as the above. > Again plot them with respect to ecutrho (Here is the image > https://pasteboard.co/GDDu3wt.png ). > From these two plots, I got confused. The convergence behaviour of > matdyn.x and ph.x at the same point is not the same. The convergence of > dispersion is much quicker ( above ecutrho=200) then convergence of > single point ph.x calc ( above ecutrho=400 ). Is it generally true? when you do a matdyn calculation on the full 4x4x4 mesh you also impose the acoustic sum rules on its fourier transform (i.e. on the real space constants) - things like making sure that the interatomic force constants are such that a translation leaves the energy in the quadratic hamiltonian exactly unchanged. when you do a single point ph.x calculation you cannot impose these symmetries/sum rules so it's all consistent - enforcing an exact condition makes your convergence faster (to be honest, I do not have a proof for this - but to give you an extreme example, if you had looked at the 3 lowest phonons at Gamma, those will always come out to be zero from a mesh calculation with acoustic sum rule imposed (as they should)), but they will typically take very high cutoffs to converge in a single point calculations (and often they do not even converge exactly to zero, due to numerical noise in the xc functional (I think)). nicola ---------------------------------------------------------------------- Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL Director, National Centre for Competence in Research NCCR MARVEL, EPFL http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
