# How to optimize the geometry of beta PHB in VASP?

I would like to calculate the elastic constants of beta PHB. From this paper. I know that it has P3221 space group. The beta PHB has upward and downward chains, which exist at one lattice site with 50% occupancy, therefore, it is really difficult to do the optimazition in VASP. The structure destroyed with using all the ISIF tags. Has anyone run into the same problem? I'm not sure I did everything right, because I am not an expert in the polymer crystallography.

More or less better results I obtained when I used ISIF=7. My structure was destroyed at the third step. The initial 2 steps gave me the reasonable geometry in CONTCAR. Please find below my INCAR file:

PREC = Accurate
ENCUT = 520
EDIFF = 1e-6
EDIFFG = -0.02
IBRION = 2
ISIF = 7
ISYM = 2
NSW = 50
ISMEAR = 0
SIGMA = 0.1
POTIM = 0.1
PSTRESS = 0.001
NPAR = 4
NSIM = 4
ALGO = Normal
IALGO = 48
ISTART = 0
LPLANE = .TRUE.
LCHARG = .FALSE.
LWAVE = .FALSE.


and POSCAR file:

New structure
1.0
9.2200002670         0.0000000000         0.0000000000
-4.6100001335         7.9847544541         0.0000000000
0.0000000000         0.0000000000         4.6599998474
C    O    H
24   12   36
Direct
0.459999949         0.990000010         0.669999897
0.010000000         0.469999939         0.336666554
0.530000031         0.540000081         0.003333231
0.990000010         0.459999949         0.330000103
0.469999939         0.010000000         0.663333416
0.540000081         0.530000031         0.996666789
0.349999934         0.970000029         0.919999897
0.029999999         0.379999936         0.586666584
0.620000064         0.650000095         0.253333241
0.970000029         0.349999934         0.080000103
0.379999936         0.029999999         0.413333446
0.650000095         0.620000064         0.746666789
0.349999934         0.930000007         0.409999967
0.070000000         0.419999927         0.076666631
0.580000043         0.650000095         0.743333280
0.930000007         0.349999934         0.590000033
0.419999927         0.070000000         0.923333347
0.650000095         0.580000043         0.256666690
0.279999942         0.089999989         0.899999857
0.910000026         0.189999953         0.566666543
0.810000062         0.720000029         0.233333185
0.089999989         0.279999942         0.100000143
0.189999953         0.910000026         0.433333486
0.720000029         0.810000062         0.766666830
0.439999938         0.009999998         0.179999948
0.990000010         0.429999948         0.846666634
0.570000052         0.560000062         0.513333261
0.009999998         0.439999938         0.820000052
0.429999948         0.990000010         0.153333381
0.560000062         0.570000052         0.486666709
0.199999973         0.829999983         0.409999967
0.170000002         0.369999975         0.076666631
0.629999995         0.800000012         0.743333280
0.829999983         0.199999973         0.590000033
0.369999975         0.170000002         0.923333347
0.800000012         0.629999995         0.256666690
0.559999883         0.119999982         0.639999866
0.879999995         0.439999908         0.306666523
0.560000122         0.440000117         0.973333180
0.119999982         0.559999883         0.360000134
0.439999908         0.879999995         0.693333447
0.440000117         0.560000122         0.026666800
0.519999862         0.909999967         0.709999859
0.090000004         0.609999895         0.376666516
0.390000135         0.480000138         0.043333191
0.909999967         0.519999862         0.290000141
0.609999895         0.090000004         0.623333454
0.480000138         0.390000135         0.956666827
0.239999965         0.840000033         0.919999897
0.159999996         0.399999976         0.586666584
0.600000024         0.760000050         0.253333241
0.840000033         0.239999965         0.080000103
0.399999976         0.159999996         0.413333446
0.760000050         0.600000024         0.746666789
0.189999968         0.059999991         0.719999909
0.939999998         0.129999980         0.386666566
0.870000005         0.810000062         0.053333241
0.059999991         0.189999968         0.280000091
0.129999980         0.939999998         0.613333404
0.810000062         0.870000005         0.946666777
0.209999964         0.089999989         0.100000024
0.910000026         0.119999975         0.766666710
0.879999995         0.790000021         0.433333367
0.089999989         0.209999964         0.899999976
0.119999975         0.910000026         0.233333305
0.790000021         0.879999995         0.566666663
0.379999936         0.219999969         0.869999886
0.780000031         0.159999967         0.536666572
0.840000033         0.620000064         0.203333214
0.219999969         0.379999936         0.130000114
0.159999967         0.780000031         0.463333458
0.620000064         0.840000033         0.796666801


Thank you in advance!

• +1. If you ran into a problem when using the ISIF tags, would you consider to show us an input file in a code block? This can substantially help you get an answer. May 27 at 7:36
• PREC = Accurate ENCUT = 520 EDIFF = 1e-5 EDIFFG = -0.02 IBRION = 2 ISIF = 3 NSW = 50 ISMEAR = 0 SIGMA = 0.1 NPAR = 4 NSIM = 4 ALGO = N ISTART = 0 LPLANE = .TRUE. LCHARG = .FALSE. LWAVE = .FALSE. can you try this and send CONTCAR in vasp chatbox May 29 at 6:28
• @pranav kumar, I did the calclulation as you sugested, however, the structure was broken, I don't understand where do you ask me to send CONTCAR? Jun 2 at 4:15
• For very large box it is difficult to conserve symmetry, I think you are trying to get elastic constants based on Trigonal symmetry. can you share some details with me such as which 7 elastic constants are you getting . I will also suggest you to use vasp chat box for further discussion chat.stackexchange.com/rooms/109983/vasp Jun 3 at 6:35