[Pw_forum] Frequency and zero point energy calculations

[oulihui666]

Dear all,
    Recently, I have been performing calulation of frequency and zero point energy of CO2 molecule. However, unfortunately, I have obtained negative frequency. According to the manual, there are several possible reseaons: 
    (1) wrong data file read.
    (2) wrong atomic masses given in input will yield wrong frequencies (but
the content of file fildyn should be valid, since the force constants, not
the dynamical matrix, are written to file).
    (3) convergence threshold for either SCF (conv thr) or phonon calculation
(tr2 ph) too large: try to reduce them.
    (4) maybe your system does have negative or strange phonon frequencies,
with the approximations you used. A negative frequency signals a
mechanical instability of the chosen structure. Check that the structure
is reasonable, and check the following parameters:
    – The cutoff for wavefunctions, ecutwfc
    – For US PP: the cutoff for the charge density, ecutrho
    – The k-point grid, especially for metallic systems!

My calculated process as follows: 
(1) Optimizing CO2 structure(input file as follows);
 &CONTROL
                 calculation = 'relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/home/olh/tmp/' ,
                  pseudo_dir = '/home/olh/pseudo/' ,
                      prefix = 'CO2' ,
 /
 &SYSTEM
                       ibrav = 1,
                   celldm(1) = 15.11781502,
                         nat = 3,
                        ntyp = 2,
                     ecutwfc = 26 ,
                     ecutrho = 260 ,
                 occupations = 'smearing' ,
                     degauss = 0.02 ,
                    smearing = 'methfessel-paxton' ,
                       nspin = 2 ,
   starting_magnetization(1) = 0.5,
   starting_magnetization(2) = 0.5,
 /
 &ELECTRONS
                    conv_thr = 1.D-12 ,
                 mixing_beta = 0.4D0 ,
 /
 &IONS
                ion_dynamics = 'bfgs' ,
 /
ATOMIC_SPECIES
    C   12.01100  C.pbe-rrkjus.UPF
    O   15.99940  O.pbe-rrkjus.UPF
ATOMIC_POSITIONS angstrom
    C      4.000000000    4.000000000    4.000000000    1  1  1
    O      5.160000000    4.000000000    4.000000000    1  1  1
    O      2.840000000    4.000000000    4.000000000    1  1  1
K_POINTS gamma
 
(2) Performing scf calculations:
 &CONTROL
                 calculation = 'scf' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/home/olh/tmp/' ,
                  pseudo_dir = '/home/olh/pseudo/' ,
                      prefix = 'CO2' ,
 /
 &SYSTEM
                       ibrav = 1,
                   celldm(1) = 15.11781502,
                         nat = 3,
                        ntyp = 2,
                     ecutwfc = 26 ,
                     ecutrho = 260 ,
                 occupations = 'smearing' ,
                     degauss = 0.02 ,
                    smearing = 'methfessel-paxton' ,
                       nspin = 2 ,
   starting_magnetization(1) = 0.5,
   starting_magnetization(2) = 0.5,
 /
 &ELECTRONS
                    conv_thr = 1.D-12 ,
                 mixing_beta = 0.4D0 ,
 /
ATOMIC_SPECIES
    C   12.01100  C.pbe-rrkjus.UPF
    O   15.99940  O.pbe-rrkjus.UPF
ATOMIC_POSITIONS angstrom
    C        4.000000000   4.000000000   4.000000000    0  0  0
    O        5.172018724   4.000000000   4.000000000    0  0  0
    O        2.827981276   4.000000000   4.000000000    0  0  0
K_POINTS automatic
  16 16 16  1 1 1
 
(3) Performing frequency calculation using PHONOS codes:
phonons of CO2 at Gamma
 &inputph
  tr2_ph=1.0d-20,
  prefix='CO2',
  epsil=.false.,
  amass(1)=12.0110,
  amass(2)=15.9994,
  outdir='/home/olh/tmp/',
  fildyn='CO2.dynG',
 /
0.0 0.0 0.0
 
Finally, I obtained the frequency of CO2 molecule, output file as follows:
    Convergence has been achieved
    Number of q in the star =    1
    List of q in the star:
         1   0.000000000   0.000000000   0.000000000
    Diagonalizing the dynamical matrix
    q = (    0.000000000   0.000000000   0.000000000 )
**************************************************************************
    omega( 1) =      -3.661345 [THz] =    -122.130124 [cm-1]
    omega( 2) =      -3.661345 [THz] =    -122.130124 [cm-1]
    omega( 3) =       3.659957 [THz] =     122.083850 [cm-1]
    omega( 4) =       7.879986 [THz] =     262.849774 [cm-1]
    omega( 5) =       7.879986 [THz] =     262.849774 [cm-1]
    omega( 6) =      19.746862 [THz] =     658.688787 [cm-1]
    omega( 7) =      19.746862 [THz] =     658.688787 [cm-1]
    omega( 8) =      38.945041 [THz] =    1299.075383 [cm-1]
    omega( 9) =      70.000609 [THz] =    2334.984491 [cm-1]
**************************************************************************
     Mode symmetry, D_4h(4/mmm) point group:
     omega(  1 -  2) =       -122.1  [cm-1]   --> E_g  X_5  M_5   R
     omega(  3 -  3) =        122.1  [cm-1]   --> A_2u X_4' M_4'  I
     omega(  4 -  5) =        262.8  [cm-1]   --> E_u  X_5' M_5'  I
     omega(  6 -  7) =        658.7  [cm-1]   --> E_u  X_5' M_5'  I
     omega(  8 -  8) =       1299.1  [cm-1]   --> A_1g X_1  M_1   R
     omega(  9 -  9) =       2335.0  [cm-1]   --> A_2u X_4' M_4'  I
 **************************************************************************
     PHONON       :     1h48m CPU time,        2h25m wall time
......
There are still negative frequency in output file by increasing K_POINTS and reducing tr2_ph, anyone who help me will be appreciated. 
In addition, I intend to calculate the zero point energy of system by PHNONS code, but it does not give the zero point energy and other thermodynamics properties in output file. How could I do? I need your help urgently.
 
Thanks in advance 
Best regards 
Lihui 

--

======================================
Lihui Ou
PH.D Candidate in Electrochemistry                      
College of Chemistry and Molecular Science            
Wuhan University,430072,Hubei Province,China 
E-mail:oulihui666 at 126.com
======================================
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