Dear PWSCF Users, I have been puzzled by the calculation of the electron-phonon (EP) coupling coefficient lambda using PWSCF for a long time. I have two questions in this issue. (1)Usually, using PWSCF to calculate EP coupling lambda, we should choose many (say one hundred or more, I am not sure. It may depend on system) q vectors in first BZ. After we finish the EP coupling calculation at all q vectors, we make an average over these q points. Then we can get the lambda value. To choose the desired q vectors in first BZ, as I know the q points can not be the zone boundary points, because it shares by other BZ. But I am not sure If the q points for high symmetry points or along high symmetry lines in the first BZ can be chosen. My question is how to choose the q points properly in the first BZ? What is the rule for this choice?
(2)I tried to calculate the lambda of BCC Nb. I got almost perfect agreement with experimental data for phonon calcualtion at several high symmetry q points (say H, P, N) using 16 16 16 k mesh. Then I use 32 32 32 kmesh to calculate the EP coulping, the calculated linewidth Gamma is quite different with experimental results. I was puzzled, It should not be this case. because phonon contribute mainly to the electron phonon coupling. So I doubt the convergency of this EP coulping calculation. By default, the pwscf uses ten step Gaussian broadening to calculate the lambda and linewidth to test the convergency. So I tried to increase the Gaussian broadening steps to see if the EP coupling calculation converges.In some steps it seems that the calculation converges.But for further steps the results go away (say several GHZ changes in Linewidth). It doesn't help even I increase the steps to 40. So My question is what is the criterion for the EP coulping convergency. Is it correct that "more the Gaussian broadening steps there are , more reliable the results are? Can someone who has any successful experiences on EP coulping lambda calculation help me? I will high appericate your help. Note that to calculate the EP coulping is one of the most powerful function in PWSCF. Best Wishes! Yanming Ma PhD Steacie Institute for Molecular Sciences, National Research Councils of Canada. 100 Sussex K1A 0R6 _________________________________________________________________ ?????????????? MSN Messenger: http://messenger.msn.com/cn
