Subhradip Ghosh wrote: > [..] I find that phonon frequencies at Gamma and [1/2 1/2 1/2] points > do not converge even after a large charge density cut-off although with > respect to k-mesh size and kinetic energy cut-off, > the convergence was achieved with reaonable values of these parameters. > [...] The lowest lying acoustic mode is not yet converged to within 1% > while the other modes have converged. > > This is very surprising because when I looked into the archive and the > relevent literature I didn't find any reference to such a behavior.
you are using ultrasoft pseudopotentials, right? the calculation of phonon frequencies - especially acoustic phonons - with USPP is quite tricky. Large cutoffs for the charge density are needed, sometimes so large to make the calculation impractical. Unfortunately I am not aware of any remedy. Last time I tried to use USPP in a phonon calculation, I gave up. I am trying again right now... Acoustic modes are sensitive to small violations of the Acoustic Sum Rule (i.e. of translational invariance), and this is well known (even in the case of norm-conserving PP, where the violation of the ASR comes from the discretization on a grid of the exchange correlation potential). At Gamma this is typically fixed by imposing the ASR on the dynamical matrix, but this may not be sufficient. Paolo -- Paolo Giannozzi, Democritos and University of Udine, Italy
