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EDIT: This issue has been resolved. The output in the DYNMAT is scaled by a parameter STEP. For IBRION = 5,6 then STEP = POTIM, while for IBRION = 7,8 then STEP = 1. The frequency difference between IBRION = 6 and IBRION = 8 also goes away if the accuracy is increased for the IBRION = 6 case.


Dear admin and vasp-users,

I'm performing calculations to determine the vibrational frequencies of bulk bcc Fe. I set up a cell with 128 atoms and use IBRION = 8. VASP correctly identifies that only one degree of freedom needs to be evaluated, in this case the x-coordinate for the 1st atom. The obtained frequencies look reasonable and if I evaluate the phonon DOS I get something that resembles experimental results.

The problem I'm having is that if I look at the DYNMAT file the entry for the x-coordinate is extremely large and the displacement is indicated to be 1.00. Since the POTIM tag doesn't affect anything when using DFPT (I've checked to be absolutely sure) I must have missed something.

I've performed calculations using IBRION = 6 and POTIM = 0.005 in order to compare. A direct comparison between the two DYNMAT files would be dependent of the POTIM value for the IBRION = 6 case, but it seems to show that IBRION = 8 gives very large values in the DYNMAT file. I also found a large difference between the frequencies with almost 0.2 THz per degree of freedom, which is not good. I expected to see a reasonable agreement between the DFPT results and the IBRION = 6 results when POTIM tends to be small. So I'm starting to question if I can trust the results for IBRION = 8.

Do anyone has a suggestion to what might be going on here? Can I improve the IBRION = 8 calculations in some way?

I have verified that the geometry is correct and that I'm using the same POTCAR, KPOINTS and POSCAR for both runs. All calculations were performed with VASP 5.2.2.


Best regards,
/Dan Fors


DYNMAT (IBRION = :
===============

1 128 1
55.847
1 1 1.0000 0.0000 0.0000
-11.948513 0.000000 0.000000
-0.020332 0.000000 0.000841
0.020933 0.000000 0.000000
-0.020332 0.000000 -0.000841
-0.020332 0.000841 0.000000
-0.061039 0.000123 0.000123
...


DYNMAT (IBRION = 6):
===============
1 128 1
55.847
1 1 0.0050 0.0000 0.0000
-0.056924 0.000000 0.000000
-0.000259 0.000000 -0.000004
0.000504 0.000000 0.000000
-0.000259 0.000000 0.000004
-0.000259 -0.000004 0.000000
-0.000211 -0.000001 -0.000001
-0.000089 0.000000 0.000000
...


INCAR (IBRION = :
==============

SYSTEM = Fe128


ENCUT = 700 eV
ENAUG = 700 eV

EDIFF = 1E-8
EDIFFG = 1E-5

LREAL = A
PREC = Accurate
ADDGRID = .TRUE.

ISMEAR = 1
SIGMA = 0.1

IBRION = 8

MAGMOM = 128*4
ISPIN = 2
LORBIT = 12
NEDOS = 1000
ALGO = Fast



INCAR (IBRION = 6):
=============

SYSTEM = Fe128


ENCUT = 700 eV
ENAUG = 700 eV

EDIFF = 1E-8
EDIFFG = 1E-5

LREAL = A
PREC = Accurate
ADDGRID = .TRUE.

ISMEAR = 1
SIGMA = 0.1

IBRION = 6
POTIM = 0.005

MAGMOM = 128*4
ISPIN = 2
LORBIT = 12
NEDOS = 1000
ALGO = Fast

KPOINTS
======

Monk
0
Monk
4 4 4
0 0 0


POSCAR:
======

bcc:
2.83310000000000
4.0065391828051933 0.0000000000000000 0.0000000000000000
0.0000000000000000 4.0065391828051933 0.0000000000000000
0.0000000000000000 0.0000000000000000 4.0065391828051933
Fe
128
Direct
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<span class='smallblacktext'>[ Edited ]</span>

Hello,
I have been doing some comparison between the IBRION = 5-6-(7 not tested yet)-8 and all vibrations are inconsistent for the three methods !!!!! It is quite frightening at that point.
I have tested it on a polymer (C H and N atoms nothing fancy) in a triclinic cell at gamma point with

INCAR
PREC = Normal
GGA =PS
ALGO = FAST
EDIFF = 1e-7
ISPIN=1
NELM = 60
ENCUT = 750
LREAL= A
ISIF=3
Ionic relaxation
EDIFFG = -5e-3
NSW = 550
IBRION = 5 (6 and
POTIM =0.01
LPLANE = .TRUE.
LSCALU = .FALSE.
NSIM = 4
DOS related values
ISMEAR = 0 ; SIGMA=0.05


----------------------------------------------------
IBRION =5 extract of OUTCAR
49 f = 68.823435 THz 432.430393 2PiTHz 2295.702601 cm-1 284.630949 meV
50 f = 68.764645 THz 432.061007 2PiTHz 2293.741586 cm-1 284.387814 meV
51 f = 65.489736 THz 411.484150 2PiTHz 2184.502402 cm-1 270.843876 meV
52 f = 65.473768 THz 411.383814 2PiTHz 2183.969736 cm-1 270.777834 meV
53 f = 48.509155 THz 304.792011 2PiTHz 1618.091196 cm-1 200.617812 meV
54 f = 48.479099 THz 304.603165 2PiTHz 1617.088641 cm-1 200.493511 meV
55 f = 48.348256 THz 303.781053 2PiTHz 1612.724187 cm-1 199.952387 meV
56 f = 48.333537 THz 303.688571 2PiTHz 1612.233211 cm-1 199.891514 meV
57 f = 48.052772 THz 301.924470 2PiTHz 1602.867888 cm-1 198.730362 meV
58 f = 48.030621 THz 301.785294 2PiTHz 1602.129026 cm-1 198.638754 meV
----------------------------------------------------
IBRION =6 extract of OUTCAR
49 f = 68.923065 THz 433.056387 2PiTHz 2299.025900 cm-1 285.042986 meV
50 f = 68.865741 THz 432.696212 2PiTHz 2297.113786 cm-1 284.805914 meV
51 f = 65.805941 THz 413.470919 2PiTHz 2195.049836 cm-1 272.151592 meV
52 f = 65.800068 THz 413.434022 2PiTHz 2194.853953 cm-1 272.127306 meV
53 f = 52.355775 THz 328.961038 2PiTHz 1746.400627 cm-1 216.526160 meV
54 f = 52.348511 THz 328.915396 2PiTHz 1746.158323 cm-1 216.496118 meV
55 f = 48.508840 THz 304.790033 2PiTHz 1618.080696 cm-1 200.616510 meV
56 f = 48.478853 THz 304.601617 2PiTHz 1617.080427 cm-1 200.492492 meV
57 f = 48.291499 THz 303.424439 2PiTHz 1610.830979 cm-1 199.717659 meV
58 f = 48.287123 THz 303.396944 2PiTHz 1610.685013 cm-1 199.699562 meV
----------------------------------------------------
IBRION =8 extracted from OUTCAR
49 f = 69.471154 THz 436.500136 2PiTHz 2317.308202 cm-1 287.309703 meV
50 f = 69.417908 THz 436.165577 2PiTHz 2315.532083 cm-1 287.089492 meV
51 f = 66.262504 THz 416.339592 2PiTHz 2210.279156 cm-1 274.039788 meV
52 f = 66.252048 THz 416.273895 2PiTHz 2209.930381 cm-1 273.996545 meV
53 f = 51.967422 THz 326.520942 2PiTHz 1733.446553 cm-1 214.920058 meV
54 f = 51.933800 THz 326.309687 2PiTHz 1732.325036 cm-1 214.781008 meV
55 f = 48.833059 THz 306.827156 2PiTHz 1628.895452 cm-1 201.957369 meV
56 f = 48.804781 THz 306.649485 2PiTHz 1627.952222 cm-1 201.840424 meV
57 f = 48.406730 THz 304.148455 2PiTHz 1614.674663 cm-1 200.194216 meV


IBRION = 6 gives an insertion of two C=C frequency in to the C-C(triple) zone compared to IBRION=5
IBRION=8 shift of at least 100 cm-1 some frquencies

Is there some kind of bug in the symmetry
HAs someone an idea to solve this problem ??

Best regards

JS

Here is the IBRION=7 to complete the study
49 f = 70.239111 THz 441.325350 2PiTHz 2342.924476 cm-1 290.485717 meV
50 f = 70.201660 THz 441.090036 2PiTHz 2341.675235 cm-1 290.330831 meV
51 f = 67.049577 THz 421.284918 2PiTHz 2236.533084 cm-1 277.294861 meV
52 f = 67.035701 THz 421.197733 2PiTHz 2236.070232 cm-1 277.237475 meV
53 f = 50.250615 THz 315.733928 2PiTHz 1676.180052 cm-1 207.819915 meV
54 f = 50.226389 THz 315.581707 2PiTHz 1675.371935 cm-1 207.719721 meV
55 f = 50.198013 THz 315.403418 2PiTHz 1674.425429 cm-1 207.602369 meV
56 f = 50.166849 THz 315.207608 2PiTHz 1673.385905 cm-1 207.473485 meV
57 f = 49.933133 THz 313.739129 2PiTHz 1665.589989 cm-1 206.506914 meV

The symmetry and no symmetry should gibe the same results ( or at least close enought) so I would agree that something looks bogus for non cubic cell
also
the difference between ibrion 5-6 is 130 cm-1
but the difference 5-7 is still 60 cm-1

I do not have much experience with vibration from response but I would have expected it to be much closer to the finite displacement ?

@jsfilhol

No offense, but I think the symmetry issuses for noncubic cells should be separated from the issue that DFPT results seem to differ from the finite difference results.

The issue I raised in the thread was that I am concerned about the magnitudes in the DYNMAT file when using DFPT. Therefore I doubt my calculations and I guess the INCAR file is lacking a crucial parameter. I have no concern with symmetry issues since IBRION = 7 gives the same result as IBRION = 8, while IBRION = 5 gives the same as IBRION = 6.

So the two issues, even though they are connected to the accuracy of the frequency calculations, must have two separate origins. User widom started a thread a couple of days ago

http://cms.mpi.univie.ac.at/vasp-forum/ ... php?3.6669

where the symmetry issues for noncubic cells were raised. Therefore I suggest we split the issues and treat them in different threads. This will also be more consistent with the thread topics.

Best regards,
/Dan


<span class='smallblacktext'>[ Edited Fri Jan 29 2010, 07:46PM ]</span>

Dear Dan

First, thank you for this topic which has clarified most of my questions on linear response.

Second, I performed calculations on silver using both finite differences and DFPT and I also observed that there is a large displacement in the DYNMAT file of the DFPT run:
1 4 1
107.868
1 1 1.0000 0.0000 0.0000
-4.447133 0.000000 0.000000
2.318145 0.000000 0.000000
-0.189157 0.000000 0.000000
2.318145 0.000000 0.000000

Of course, resulting phonon frequencies are largely different in both cases.

I think that in order to determine whether IBRION=8 works, we should try to compare the value of the second derivative of the total energy in both calculations. They should be similar if the calculation precision is high enough.

Problem is, I don't know where to find them...

Regards