[ase-users] Comparison of Vibrational Frequencies Obtained from VASP and ASE

[Kondov, Ivan (SCC)]

Dear Yu-Ren Chen,

There are many reasons to have this mismatch. The methods used both in VASP and in ASE base on numerical finite-difference calculation of the Hessian matrix. I can guess the following things:

1. The displacements given by POTIM = 0.015 is probably only effective in the VASP ibrion=5 calculation. If you have used the Vibrations object from the ase.vibrations module the relevant parameter is called ‘delta’ and, to have a fair comparison, it should also be set to 0.015. Apart from this a displacement of 0.015 Angstrom might not be tight enough to converge the numerical derivatives.

2. The energy convergence criterion (EDIFF = 1.00e-04) is maybe too loose. Usually at least EDIFF = 1e-6 or even EDIFF = 1e-7 should be set to have sufficiently accurate forces. The convergence of frequencies with POTIM (resp. ‘delta’ parameter in the ASE Vibrations class) and EDIFF has to be systematically checked.

3. In principle, the structure should be relaxed before comparison. The imaginary frequencies may indicate non-minimum structure but also non-converged force derivatives (see above).

If the converged results still differ too much then this might be a problem in ASE. But this is unlikely because in your calculation the highest three frequencies, that are relevant for H2O, agree within 3%. The other six are translations and rotations and should be ideally zeros.

Best regards,
Ivan



Von: ase-users-bounces at listserv.fysik.dtu.dk <ase-users-bounces at listserv.fysik.dtu.dk> im Auftrag von 陳佑任 via ase-users <ase-users at listserv.fysik.dtu.dk>
Gesendet: Montag, 22. Mai 2017 10:35
An: ase-users at listserv.fysik.dtu.dk
Betreff: [ase-users] Comparison of Vibrational Frequencies Obtained from VASP and ASE
  

Dear ASE users,
 
I am using ASE to do a normal mode analysis. However, I have a problem that the frequencies given by ASE is not consistent with the frequencies given by the normal mode analysis  implemented in VASP (IBRION = 5) although I think I have used the same settings. The settings are shown below.
 
--------------------------
POSCAR used by both ASE and VASP:
 
H O
   1.00000000000000
    10.0000000000000000    0.0000000000000000    0.0000000000000000
     0.0000000000000000   10.0000000000000000    0.0000000000000000
     0.0000000000000000    0.0000000000000000   10.0000000000000000
   H    O
     2     1
Selective dynamics
Cartesian
        5.1707077079      4.0137770615      5.2080185115     T   T   T
        5.0954999575      5.5514905157      5.1493322874     T   T   T
        5.5902191105      4.8193934419      5.5566036301     T   T   T
 
  0.00000000E+00  0.00000000E+00  0.00000000E+00
  0.00000000E+00  0.00000000E+00  0.00000000E+00
  0.00000000E+00  0.00000000E+00  0.00000000E+00
 
 
---------------------------------
 
INCAR generated by ASE:
INCAR created by Atomic Simulation Environment
 ENCUT = 300.000000
 PARAM2 = 1.000000
 AGGAC = 0.000000
 PARAM1 = 0.123400
 SIGMA = 0.010000
 POTIM = 0.015000
 EDIFF = 1.00e-04
 EDIFFG = 5.00e-02
 SYSTEM = H2O
 GGA = MK
 ALGO = fast
 ISYM = 0
 ISPIN = 2
 ISMEAR = -1
 ISTART = 1
 IALGO = 38
 NELM = 60
 NSW = 1
 IBRION = -1
 NPAR = 4
 IDIPOL = 4
 DIPOL = 0.5000 0.5000 0.5000
 LUSE_VDW = .TRUE.
 LDIPOL = .TRUE.
 LREAL = .FALSE.
 
----------------------------------------
 
INCAR used for the normal mode analysis implemented in VASP
 
ISMEAR = -1     
SIGMA  = 0.01
EDIFF  = 1.d-4
EDIFFG = -0.05
ISPIN  = 2
ALGO   = fast
NSW    = 600
ISIF   = 0 
IBRION = 5
LREAL  = .F.
IALGO  = 38
NELM   = 60
ENCUT  = 300
ISYM   = 0
NPAR   = 4
ISTART = 1
IDIPOL = 4
LDIPOL = T
DIPOL = 0.5 0.5 0.5
POTIM = 0.015
GGA = MK
PARAM1 = 0.1234
PARAM2 = 1.0000
LUSE_VDW = .TRUE.
AGGAC = 0.0000
 
-------------------------
 
The results are shown below:
 
VASP:
1 f  = 3677.961907 cm-1   
2 f  = 3587.067891 cm-1   
3 f  = 1568.669032 cm-1   
4 f  =   265.535571 cm-1    
5 f  =   234.019931 cm-1    
6 f  =   121.480968 cm-1    
7 f/i=       8.023132 cm-1     
8 f/i=     19.960428 cm-1    
9 f/i=   309.311267 cm-1  
 
ASE:
#   mev      cm-1
0   23.0i    185.3i
1   12.9i    104.3i
2    2.3i     18.8i
3   10.1      81.6
4   27.3     220.5
5   32.2     259.8
6  199.1    1605.8
7  436.9    3523.6
8  457.0    3685.7
 
As we can see from the results, the difference between the two calculations is not small. Does anyone have an idea where this mismatch may come from, or whether this is within  the range of the error of the method?


Yu-Ren Chen


Institute of Atomic and Molecular Sciences, Academia Sinica