[Pw_forum] Negative frequencies in phonon calculation

[Alberto Milani]

Dear Stefano,
Thank for your advice.    
Here below I attach the final part of the output where stresses are computed 
for my relaxed system. It seems that the computation satisfy the 0.5 Kbar 
threshold on total stress (this threshold has been mentioned somewhere in the 
forum).
Is it possible that for such a system these residual stress are still too 
large and produce nasty effects on the TA phonons?
Thank you
yours,
A. M.   

the Fermi energy is    -4.5276 ev

!    total energy              =   -22.64532713 ryd
     estimated scf accuracy    <        1.1E-09 ryd

     band energy sum           =    -7.45543717 ryd
     one-electron contribution =   -66.15073880 ryd
     hartree contribution      =    33.76544381 ryd
     xc contribution           =    -6.79656224 ryd
     ewald contribution        =    16.53653010 ryd

     convergence has been achieved

     Forces acting on atoms (Ry/au):

     atom   1 type  1   force =     0.00000000    0.00000000    0.00000339
     atom   2 type  1   force =     0.00000000    0.00000000   -0.00000339

     Total force =     0.000005     Total SCF correction =     0.000065

    entering subroutine stress ...

          total   stress  (ryd/bohr**3)                  (kbar)     P=   -0.21
  -0.00000081   0.00000000   0.00000000         -0.12      0.00      0.00
   0.00000000  -0.00000081   0.00000000          0.00     -0.12      0.00
   0.00000000   0.00000000  -0.00000266          0.00      0.00     -0.39

     kinetic stress (kbar)    845.28      0.00      0.00
                                0.00    845.28      0.00
                                0.00      0.00    573.85

     local   stress (kbar)    206.31      0.00      0.00
                                0.00    206.31     -0.00
                                0.00     -0.00  -7200.10

     nonloc. stress (kbar)    251.24      0.00      0.00
                                0.00    251.24      0.00
                                0.00      0.00    288.31

     hartree stress (kbar)      0.57     -0.00      0.00
                               -0.00      0.57     -0.00
                                0.00     -0.00   2866.06

     exc-cor stress (kbar)   -183.25     -0.00      0.00
                               -0.00   -183.25      0.00
                                0.00      0.00   -173.28

     corecor stress (kbar)      0.00      0.00      0.00
                                0.00      0.00      0.00
                                0.00      0.00      0.00

     ewald   stress (kbar)  -1120.28      0.00      0.00
                                0.00  -1120.28      0.00
                                0.00      0.00   3644.76

     hubbard stress (kbar)      0.00      0.00      0.00
                                0.00      0.00      0.00
                                0.00      0.00      0.00

     bfgs converged in  10 scf cycles and   7 bfgs steps

     End of BFGS geometry calculation

     Final energy            =     -22.6453271292 ryd

     Saving the approximate inverse hessian


CELL_PARAMETERS (alat)
   1.000000000   0.000000000   0.000000000
   0.000000000   1.000000000   0.000000000
   0.000000000   0.000000000   0.256710000

ATOMIC_POSITIONS (angstrom)
C        0.000000000   0.000000000  -0.632225902
C        0.000000000   0.000000000   0.632225902



     Writing file        cc.save for program phonon