MD simulation NPT external pressure

[t.liu]

Dear all,

I want to do the MD simulation with fix T and P by IBRION = 0, and the the INCAR file is as following,

ISTART = 0
INIWAV = 1

ISYM = 0

ENCUT = 800
ENAUG = 2400
PREC = high
MAXMIX = 40
EDIFF = 0.001
NELMIN = 4

ISMEAR = -1
SIGMA = 0.172

IBRION = 0
LREAL = .FALSE.

ALGO = Very Fast

NSW = 1000

LCHARG = .FALSE.
LWAVE = .TURE.
TEBEG = 2000.0
TEEND = 2000.0

SMASS = 0
NBLOCK = 50
POTIM = 1.0

PSTRESS = 1500
ISIF = 3

PMASS = 20

DMALGO = 3
LANGEVIN_GAMMA = 50,50,50
LANGEVIN_GAMMA_L = 50


In the OUTCAT file, the stress is about 4000 kB by grep 'in kB' after hundreds of steps

in kB 4172.66112 3902.39396 4400.48371 0.75775 0.00033 -0.00129
in kB 4175.23568 3903.55126 4397.35739 -1.09890 4.17513 0.79122
in kB 4177.58431 3902.41118 4388.94827 -1.87095 8.17570 1.01140
in kB 4180.84173 3901.55583 4377.15279 -2.23297 11.91227 0.64589
in kB 4183.17536 3899.89882 4361.25600 -1.97631 15.42282 -0.17552
in kB 4184.61089 3897.95280 4341.75406 -1.03387 18.65903 -1.31153
in kB 4184.97713 3895.89317 4319.14836 0.60726 21.60255 -2.56697
in kB 4183.96156 3893.68236 4293.78485 2.78659 24.21068 -3.74790
in kB 4181.50387 3891.18446 4266.20603 5.28728 26.42985 -4.67963
in kB 4177.77326 3888.18529 4237.08118 7.81475 28.20301 -5.22024
in kB 4173.13620 3884.51263 4207.21849 10.00879 29.47044 -5.27320


My question is why I apply PSTRESS = 1500 kB, but it shows about 4000kB for XX YY ZZ, in the OUTCAR?


I also tried the normal structure relaxation, with IBRION = 2, and with all the other parameters like in the INCAR file above, the stress shown in the OUTCAR file is correct as following,

in kB 4164.18638 3891.83071 4390.16931 -0.00023 0.00009 0.00002
in kB 3839.38224 3590.92924 4038.54882 0.00163 -0.00007 -0.00002
in kB 2996.27502 2807.91069 3130.59147 -0.00024 -0.00002 -0.00013
in kB 1778.69087 1669.38225 1831.87127 -0.00002 0.00001 -0.00004
in kB 1497.67446 1404.55014 1534.81292 0.00048 -0.00002 0.00006
in kB 1448.12627 1357.76865 1482.58460 0.00035 0.00002 -0.00004
in kB 1449.96058 1361.96624 1487.14790 0.00273 0.00009 0.00010
in kB 1456.56412 1374.89207 1499.80704 0.00234 -0.00000 0.00001
in kB 1477.63406 1406.94473 1524.00978 -0.00029 -0.00005 0.00003
in kB 1548.88679 1500.03893 1572.79441 -0.00230 -0.00005 0.00007
in kB 1803.80977 1850.93418 1743.05711 -0.00311 0.00001 0.00025

....

in kB 1500.08061 1499.76833 1499.91247 0.00031 -0.00003 -0.00000
in kB 1499.86822 1499.56459 1499.69498 -0.00016 -0.00000 0.00002
in kB 1499.96526 1499.79712 1500.02728 0.00052 -0.00003 0.00000
in kB 1500.17449 1500.27199 1500.67246 0.00043 -0.00001 -0.00002
in kB 1499.88523 1500.04832 1500.34787 -0.00023 -0.00001 -0.00003
in kB 1499.30292 1499.59149 1499.70567 -0.00008 -0.00000 -0.00003
in kB 1499.58503 1499.88517 1499.85329 -0.00014 0.00002 -0.00004
in kB 1500.29745 1500.58586 1500.17384 -0.00009 0.00002 0.00000
in kB 1500.10129 1500.25323 1499.81734 0.00016 0.00004 0.00002
in kB 1500.08360 1500.17931 1499.73520 0.00020 0.00001 0.00001
in kB 1500.07581 1500.15166 1499.90178 0.00048 0.00001 -0.00001
in kB 1500.07394 1500.13570 1500.00074 0.00028 0.00001 0.00000


I noticed that the starting stress is 4164 kB as MD simulation, but it is going down to the right stress 1500 kB very quick. Does it mean that My MD simulation step is not enough long or there is some parameters set wrong in the MD simulation?

Thanks.
Tao

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