Why VASP calculates the elastic constant for another trigonal space group?

Please, help me to understand the problem. I calculated the elastic constants for the beta PHB (Space group P3221 (154)), it has to show the reduced elastic constant matrix (six), however, VASP calculated me 7 constants for the trigonal symmetry point group 3 (I used IBRION=6, ISIF =3, NFREE=2 and POMIT =0.015). I really don't have any idea why maybe anyone has the same problem? I attached my POSCAR file of beta PHB below. Thank you in advance!

My iNCAR:

PREC    = Accurate
ENCUT   = 520
EDIFF   = 1e-6
IBRION  = 6
NFREE   = 2
ISIF    = 3
ISYM    = 2
POTIM   = 0.03
NSW     = 1
ISMEAR  = 0
SIGMA   = 0.1
ALGO    = Normal
IALGO   = 48
ISTART  = 0
LPLANE  = .TRUE.
LCHARG  = .FALSE.
LWAVE   = .FALSE.


My POSCAR :

betaPHB
1.00000000000000
9.8451449602803063   -0.0000000000135996    0.0000000000000000
-4.9225724801283759    8.5261456395240547    0.0000000000000000
0.0000000000000000    0.0000000000000000    4.7787902416829580
C    O    H
12     6    18
Direct
0.4473348233320940  0.0098405107170283  0.6760647908616504
-0.0098405007170275  0.4374943136150696  0.3427314468616541
0.5625056563849357  0.5526651466679172  0.0093981238616490
0.3366105966273176  0.9865190814127033  0.9194381388236403
0.0134809465872924  0.3500915452146104  0.5861048248236396
0.6499084547853901  0.6633893733726792  0.2527714818236401
0.3529941479661700  0.9450538256463469  0.4095367091398003
0.0549461813536503  0.4079403223198091  0.0762033731398051
0.5920596476801820  0.6470058220338273  0.7428700221398052
0.2595370428953800  0.0867656901155276  0.8956333322755682
0.9132342808844670  0.1727713537798490  0.5623000192755711
0.8272286462201509  0.7404629871046158  0.2289666612755733
0.4289822938404916  0.0307095082417344  0.1799468951856006
0.9692905017582659  0.3982727945987476  0.8466135811855999
0.6017272054012526  0.5710177061595080  0.5132802081855978
0.2217465615454436  0.8287633736506727  0.4032896651433156
0.1712366113493296  0.3929831728947801  0.0699563291433201
0.6070168271052196  0.7782534534545532  0.7366229781433196
0.5360727369674801  0.1339552292303300  0.6484256472182774
0.8660447627696694  0.4021175007371532  0.3150923032182820
0.5978824992628468  0.4639272630325195  0.9817589602182746
0.5084713038609513  0.9437122479440904  0.7170050087321858
0.0562877230559037  0.5647590559168535  0.3836716647321826
0.4352409730831384  0.4915286961390490  0.0503383407321812
0.2476974100787215  0.8620678575193049  0.9347039188362016
0.1379321714807079  0.3856295665594249  0.6013706048362009
0.6143704334405750  0.7523025749212737  0.2680372618362011
0.1875979130778683  0.0556088716245804  0.7051825810070412
0.9443911253754161  0.1319890374532876  0.3718492370070449
0.8680109625467131  0.8124020869221317  0.0385159130070434
0.1827452577710241  0.0681168022337463  0.0754234213029564
0.9318831687662399  0.1146284565372744  0.7420901073029557
0.8853715284627206  0.8172547562289704  0.4087567643029560
0.3478634616025427  0.2118811433998376  0.8853498234737521
0.7881188266001669  0.1359823182027122  0.5520165094737514
0.8640177107972945  0.6521365383974570  0.2186831514737536

• May you can format your INCAR and POSCAR into the code block. – Jack Apr 29 at 11:19
• Please put your INCAR and POSCAR into a code block as Jack suggested. – Nike Dattani Apr 29 at 13:31
• Your POSCAR is little tilted in xy direction. If you have calculated elastic contants, you can see that two elastic constants are nearby same with some difference. Just verify your space group using spglib python library (best use vaspkit). – pranav kumar Apr 29 at 16:55
• I have check your POSCAR, it's space group is 145. It is mainly due to relaxation due large cell using ISIF3 lost little symmetry – pranav kumar Apr 29 at 17:06
• pranav kumar, thank you, I think my fixed-pressure optimization with ISIF=3 and then the fixed-volume optimization with the ISIF=2 led to the space group number changing. Could you please give me advice, is it better to use ISIF=7 during the optimizaton and after that for the elastic constan calculation? I'm really new in this. – Irina Apr 29 at 18:35

Elastic constants can be calculated using two approach, one with Energy-strain relationship and another with stress-strain relationship. The key ingredient in above two methods is energy or stress at particular mode of distortion. For example in cubic crystal which have 3 fold symmetry about body diagonal need three independent elastic parameters to define second-order elastic constants. VASP algorithm IBRION=6 also do the same. Some time problem of not getting elastic constants correctly depends on many parameters. Specially with large systems which consists of many number of atoms along with different atomic numbers, full volume relaxation results in difference between input and output structure. For example you can try relaxation of 4 atom aluminum unit cell and re-relaxed its supercell again with same K-space density and kinetic energy cutoff. You may get minor difference in output configuration with ISIF=3(full volume and position relaxation). You can also simply check your input file symmetry using python spglib library or just use vaspkit. The question arises if there is minor change in symmetry, can we get same number of elastic constants as it should be. The answer is check your elastic constants values, is two elastic constants are apart by small amount, you can always say those are equal. Other way to calculate elastic constants without VASP andIBRION=6 is just distort crystal manually based on symmetry and do only position relaxation(ISIF=2) and then fit energy and distorsion by second order polynomial.