Problem with vibrational frequencies calculation

[duvalmbongo]

Dear all,

I need help!

I am calculating the diffusion pre-factor of He-diffusion in fluoro-apatite
unit cell. To do so, I did the NEB and CI-NEB calculation to find
activation barrier energy and the MEP pathways as explained in the VTSTTcode. Now I am
calculating the vibrational frequency of He atom in the transtion state
(saddle point) using a Dynamical Matrix tool. The problem is that all three
frequencies are nul (zero). I was expecting to find two real ones and an
imaginary one. Here are my
input files:

INCAR:

LWAVE = .FALSE.
LREAL = .TRUE.
PREC = Med
GGA = PE
ALGO = Fast
ENCUT = 500
EDIFF = 1E-6
EDIFFG = -0.0001
ISMEAR = 0
SIGMA = 0.3
ISIF = 2 # volume constant
#Dynamical matrix setup
ICHAIN=1
IBRION = 3
POTIM = 0.0
NSW = 4 # NSW=(DOF/M)+1

DISPLACECAR: Only He atome is displaced (atom number 43)

0 0 0 1 4.03538511758153
0 0 0 2 4.18154935367412
0 0 0 3 4.23680139145379
0 0 0 4 5.08857263756467
0 0 0 5 2.48764135351475
0 0 0 6 5.35366370819887
0 0 0 7 7.52370357730919
0 0 0 8 4.40287322830062
0 0 0 9 3.03998734127859
0 0 0 10 2.08890839455547
0 0 0 11 2.57662773101188
0 0 0 12 4.74869164919919
0 0 0 13 4.67998811951044
0 0 0 14 3.33174123777092
0 0 0 15 6.33597091844552
0 0 0 16 2.71124726858289
0 0 0 17 2.26600382548488
0 0 0 18 3.55151267370887
0 0 0 19 3.84352124039211
0 0 0 20 3.93779899037237
0 0 0 21 3.93370996128381
0 0 0 22 3.82952391507901
0 0 0 23 4.28829922969097
0 0 0 24 4.09909254111913
0 0 0 25 6.21282951623158
0 0 0 26 4.48337350352496
0 0 0 27 2.1480354127115
0 0 0 28 4.91954605155162
0 0 0 29 3.2894703291538
0 0 0 30 4.57884581270151
0 0 0 31 6.14543927643163
0 0 0 32 3.70289027165505
0 0 0 33 7.35602602740697
0 0 0 34 2.03107954933063
0 0 0 35 4.60800032685474
0 0 0 36 6.5260584699008
0 0 0 37 2.70827735668947
0 0 0 38 2.08929541278468
0 0 0 39 5.00195631868283
0 0 0 40 4.99819422958791
0 0 0 41 3.64043603946682
0 0 0 42 3.43702473722405
0.005 0.005 0.005 43 0


KPOINTS

He in ApF unitcell
0
Monkhorst-Pack
3 3 3
0 0 0

POSCAR

He atom in site S2 of Ca10(PO4)6F2 unitc
1.00000000000000
9.4808387799999991 0.0000000000000000 0.0000000000000000
-4.7404191600000001 8.2106467599999995 0.0000000000000000
0.0000000000000000 0.0000000000000000 6.9309554100000001
10 6 24 2 1
Direct
0.6719413909923517 0.3491449058633762 0.0097694639181682
0.9897791696595561 0.2418274934401623 0.2464312349144251
0.3397504901151720 0.6784743154616777 0.4950185960898779
0.3214216343754188 0.6497411647984279 0.9948017370423443
0.6639544615145709 0.3207420470472044 0.5063038289881163
0.7438234491360447 0.9951856607297813 0.7562799271050835
0.7643041191894255 0.7601319251517253 0.2588702806073897
0.0056348067672261 0.7675702899314558 0.7560574227484409
0.2550049313109710 0.0002394744625818 0.2293846263405754
0.2199292108347279 0.2562706991353778 0.7599198410811141
0.3635661333507585 0.4025563765915123 0.2461046769650324
0.9663448848752623 0.3714354181697104 0.7440349883226816
0.6066251582550137 0.9759096852235558 0.2436645754227200
0.6333919000052148 0.6061796693811417 0.7591510123544793
0.0256649599813699 0.6330386870030580 0.2573556901555847
0.3977400843380536 0.0140090228107397 0.7394486822587467
0.4667916419784675 0.3166420157971785 0.2422217609503150
0.4726442714468675 0.5912579132222078 0.2469280578690487
0.2529212167475734 0.3543965752994183 0.0639490144688111
0.1534184146802886 0.4840593465530887 0.7326328292432152
0.6726881985663269 0.1624536386295290 0.2461392531098667
0.5169798488743780 0.6768299749581246 0.7584591730888164
0.8381286042345408 0.5186093073358933 0.2650782640798184
0.3308982374736118 0.8289989143836891 0.7641353309116568
0.8767186442222439 0.4704096779100624 0.7610256298332985
0.4201526002095335 0.8775456420911851 0.2028435376607334
0.5351826241201962 0.4166986129902384 0.7674575489059876
0.1156612200633020 0.5338769399443066 0.2470312143366371
0.5803200759887019 0.1193302718288578 0.8011613269442748
0.8974346520288208 0.2607409900690882 0.5626078900609670
0.7000136574908424 0.9378601118276677 0.0877418016687415
0.7431756182029545 0.6625117479810783 0.5770302116508645
0.0741414123044279 0.7452311084033099 0.0773031410088905
0.3929066594041970 0.0482390757514892 0.5236282620389403
0.7490544041956824 0.6671698249603537 0.9370153938728691
0.0894775795620504 0.7459858874288798 0.4366149168562998
0.2918058165985471 0.0607287592653236 0.8673088083480067
0.2473309326121225 0.3536534945479701 0.4220265743737883
0.9251986273197730 0.2565665999474689 0.9200086397778122
0.6376733047299012 0.9143675011492007 0.4388588256827433
0.0155086194938860 0.0090050998648579 0.2572950887945993
0.9894235757980141 0.0059195233649092 0.7247769925505665
0.3762862436442447 0.2528013231523119 0.5500330233937589

Thank you in advanced for your help

Duval

[graeme]

Check to see that vasp ran for the 4 iterations; if it didn't, lower EDIFFG, even to zero. Also, make sure that the vtstcode is linked into vasp. Then, I would check the forces and the Hessian matrix that is produced. I suspect some technical problem, however, because even if the force differences are not accurate, you should not get precisely zero modes.

[duvalmbongo]

Dear Prof Graeme,
Thank you for your reply,

VASP has ran the 4 iterations, here is the my OSZICAR file:

N E dE d eps ncg rms rms(c)
DAV: 1 0.119767145293E+04 0.11977E+04 -0.17463E+05 6720 0.117E+03
DAV: 2 -0.320113797180E+03 -0.15178E+04 -0.15124E+04 10080 0.214E+02
DAV: 3 -0.334084640930E+03 -0.13971E+02 -0.13971E+02 6720 0.302E+01
DAV: 4 -0.334157411772E+03 -0.72771E-01 -0.72771E-01 13440 0.189E+00
DAV: 5 -0.334157421550E+03 -0.97775E-05 -0.97775E-05 6720 0.191E-02 0.378E+01
RMM: 6 -0.299922076963E+03 0.34235E+02 -0.10767E+02 6720 0.157E+01 0.120E+01
RMM: 7 -0.299449580835E+03 0.47250E+00 -0.15974E+00 6738 0.237E+00 0.755E+00
RMM: 8 -0.299500594110E+03 -0.51013E-01 -0.10708E+00 6721 0.255E+00 0.131E+00
RMM: 9 -0.299467224324E+03 0.33370E-01 -0.16578E-01 6816 0.645E-01 0.365E-01
RMM: 10 -0.299475752742E+03 -0.85284E-02 -0.29556E-02 7029 0.260E-01 0.261E-01
RMM: 11 -0.299476260322E+03 -0.50758E-03 -0.66355E-03 7329 0.135E-01 0.608E-02
RMM: 12 -0.299476405606E+03 -0.14528E-03 -0.11224E-03 7180 0.541E-02 0.269E-02
RMM: 13 -0.299476405410E+03 0.19582E-06 -0.22960E-04 9822 0.203E-02 0.972E-03
RMM: 14 -0.299476414644E+03 -0.92341E-05 -0.53333E-05 9887 0.964E-03 0.393E-03
RMM: 15 -0.299476416211E+03 -0.15669E-05 -0.16570E-05 9714 0.548E-03 0.358E-03
RMM: 16 -0.299476417305E+03 -0.10942E-05 -0.48332E-06 6510 0.302E-03 0.141E-03
RMM: 17 -0.299476417619E+03 -0.31360E-06 -0.15589E-06 4924 0.185E-03
1 F= -.29947642E+03 E0= -.29947642E+03 d E =-.299476E+03
N E dE d eps ncg rms rms(c)
DAV: 1 -0.299476417948E+03 -0.29948E+03 -0.13787E-06 6720 0.138E-03 0.913E-04
RMM: 2 -0.299476418032E+03 -0.83706E-07 -0.17202E-10 1764 0.258E-06
2 F= -.29947642E+03 E0= -.29947642E+03 d E =-.299476E+03
N E dE d eps ncg rms rms(c)
DAV: 1 -0.299476418154E+03 -0.29948E+03 -0.18386E-08 6720 0.320E-04 0.132E-04
RMM: 2 -0.299476418143E+03 0.11139E-07 0.00000E+00 448 0.000E+00
3 F= -.29947642E+03 E0= -.29947642E+03 d E =-.299476E+03
N E dE d eps ncg rms rms(c)
DAV: 1 -0.299476418134E+03 -0.29948E+03 -0.26733E-09 13440 0.109E-04 0.554E-05
RMM: 2 -0.299476418134E+03 0.37107E-09 0.00000E+00 111 0.000E+00
4 F= -.29947642E+03 E0= -.29947642E+03 d E =-.299476E+03

PS: I am running vasp.4.6.28 version

[duvalmbongo]

Dear Prof Graeme,
I use VTST version 3.0b which was compile as describ on VTSTT site and it is recognise by VASP.
I attached the OUTCAR file of my calculation.

[graeme]

It looks like our bug. If you have access to the code, can you check line 137 in chain.F and see if it is commented:

! IF (images==0) RETURN

If it is not, it should be, as in the code on http://theory.cm.utexas.edu/vtsttools/code/

Perhaps there was some problem with the version increment which this bug; I'm going to switch our version number to 3.0c to be safe.

[duvalmbongo]

Dear Prof Graeme,

Yes it was a bug
I download the new version and compile.

Now I get real frequencies but still have a problem because there is no imaginary frequency. As I said yersterday It is a saddle point, so I expect one imaginary frequency.
Here are the 3 frequencies:

339.845608 cm^{-1} ... 0
530.596435 cm^{-1} ... 0
716.414534 cm^{-1} ... 0

Thank you

[graeme]

Ok, this is a good start. These frequency calculations are very sensitive. You can find a number of discussions about this on the forum here. Make sure that you have a small ediff and accurate forces (prec=high and lreal=.false help, but should not be essential). More importantly, make sure to include all of the relevant atoms. Although the He may be moving the most, you can not be sure that its motion is not coupled to its neighbors. But anyway, read the discussions about this, and check for convergence.