[Pw_forum] questions about the phonon calculation

[Jia-An Yan]

Dear PWscf users,

I have some questions during my phonon calculations using PWscf 3.2.2.

1) If I switch the iverbosity=.true. in the input file, the output of
ph.xwill list  the eigenvectors for each mode before entering the scf
iterations. If I understand correctly, the code can determine an initial
displacement patterns even WITHOUT solving the linear system. I looked into
the source code and found that this is done in the set_irr.f90 subroutine.
The comment line says:

!    This subroutine computes a basis for all the irreducible
!     representations of the small group of q, which are contained
!     in the representation which has as basis the displacement vectors.
!     This is achieved by building a random hermitean matrix,
!     symmetrizing it and diagonalizing the result. The eigenvectors
!     give a basis for the irreducible representations of the
!     small group of q.

I noticed that the calculation of the first-order perturbation dVscf also
needs the initial displacement pattern for each mode to calculate the
contribution of the local potential (in dvqpsi_us.f90).  Can anybody here
provide me more details about the algorithm? thanks

2)  I try to output the dVscf for each mode from the solve_linter.f90 after
the convergence has been reached. However, it seems that the output value
will change when I increase the Ecut (the FFT grid and also the symmetry
operations will be different comparing with previous case). In fact, in all
cases the Ecut has been already converged.  I believe there should be
something missed in my output of dVscf. Can anybody here give me some hints
about that?  Thanks!

Best,
Jia-An Yan

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School of Physics,
Georgia Institute of Technology
837 State Street,
Atlanta, Georgia 30332
U.S.A.
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