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[Tianshu Li <ts_li@berkeley.edu>]

Greetings,

I have been suffering from the convergence problem when dealing with the phonon calculation in the transition metal system, say, Vanadium. The symptom I can describe is that the convergence algorithm (CG, Anderson mixing and simple mixing) poorly behaves (it oscillates in CG while totally blows out in Anderson and simple mixing) when the wavevector q is close to the Gamma point. On the other hand, they work fine for the shorter wavelength phonons. I tried something I can think of to improve the convergence behavior, including the different pseudopotential schemes, higher energy cutoff, denser kpoints, more strict convergence tolerance on GS, but they don't seem to help much. (Here comes another issue that the high energy cutoff along with the dense kpoints would dramatically increase the NSC wavefunction storage to 2GB quota that can't be read by the following SC step. How would anybody solve that dilemma?) The other thing I might also try is to raise the number of bands, but given the fact that there are only 5 electrons and as many as 14 bands have already been involved, I can hardly see how that would help. From exploring the former archives regarding the similar issues, I know the exchange-correlation and pseudopotential schemes are particularly important for the quality of phonon calculations. But I am not sure how the convergence behavior would rely on the wavevector q. Do I just miss something critical? Can anybody help me out on this issue? Many thanks.

Tianshu Li
DMSE
University of California, Berkeley



Here are the input parameters (Run on MPI Linux clusters)

(LDA+FHI(TM type) pseudopotential)

INPUT
##########################################################
   ndtset  3

#Data set 1: Ground state calculation
  kptopt1   1          # Automatic generation of k points, taking
                            # into account the symmetry
    iscf1   5            # CG
  prtden1   1             # Will be needed for dataset 2
  tolvrs1   1.0d-20       # SCF stopping criterion


#Data set 2: Non-self consistent ground-state calculation
# with a general q [-0.05 0.05 0.05] along Gamma-X in BCC

    nqpt2   1
     qpt2   -0.05 0.05 0.05
  getwfk2   1             # Uses as input wfs the output wfs of the dataset 1
  getden2   1             # Uses as input density the output density of the dataset 1
  tolwfr2   1.0d-20       # Stopping tolerance in residual wfn (pretty strict)
  kptopt2   3             # No symmetry taken into account
                        
    iscf2   -2            # Non-self-consistent calculation

#Data set 3: RF calculation

  rfphon3   1             # Activate the calculation of the atomic dispacement perturbations
 rfatpol3   1 1           # All the atoms will be displaced
   rfdir3   1 1 1         # x, y, and z

    nqpt3   1
     qpt3   -0.05 0.05 0.05
  getwfk3   1             # Uses as input wfs the output wfs of the dataset 1
  getwfq3   2             # Uses as input ddk wfs the output of the dataset 2

  kptopt3   3           
  tolvrs3   1.0d-10     # SC
                       
    iscf3   5             # CG

# Common input variables

ecut    50.0          
nstep  70              # Maximal number of SCF cycles

acell 3*5.6020181037 
rprim   -0.5  0.5  0.5  # BCC
         0.5 -0.5  0.5
         0.5  0.5 -0.5


kptrlatt  0  8  8      # Tried denser kpoints
          8  0  8     
          8  8  0     
                      


ixc 7                 # Perdew-Wang 92 functional, consistent with
                        # FHI potential (TM scheme)
occopt 3            # Fermi-Dirac smearing
tsmear 0.01       # Typical for transition metal
nband  20          # For phonon calculation, use large number of
                         # band.

ecutsm  0.5             # Energy cutoff smearing

ntypat 1              
                    
znucl  23            # V

natom 1                # There is only one atom V
typat 1            
xred                    # This keyword indicate that the location of the atoms
                           # will follow, one triplet of number for each atom
   0.0  0.0  0.0       # Triplet giving the REDUCED coordinate of atom 1.