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Re: [lammps-users] Strange behavior of pressure
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Re: [lammps-users] Strange behavior of pressure


From: Sandra Cristina Costa Prado <sandra.prado01@...6996...>
Date: Tue, 18 Jul 2017 17:26:57 +0000


Dear Axel, the question is:

why in a NPT simulation the pressure which is supposed to be 4.5 GPa falls to 2.5 GPa (first structural transition)?


Thanks,

Sandra





De: Axel Kohlmeyer <akohlmey@...24...>
Enviado: segunda-feira, 17 de julho de 2017 17:48:57
Para: Sandra Cristina Costa Prado
Cc: lammps-users@lists.sourceforge.net
Assunto: Re: [lammps-users] Strange behavior of pressure
 
On Mon, Jul 17, 2017 at 9:28 AM, Sandra Cristina Costa Prado
<sandra.prado01@...6996...> wrote:
> Dear lammps users,
>
> I performed lammps simulations in order to observe structural transformation
> induced by pressure in BaS. When the transition pressure is reached, the
> volume should show a remarkable fall while the pressure contuinues to
> increase. Instead of this, the volume shows a small fall, while the pressure
> decrease substantially. After this, the remakable fall in the volume occurs
> and the pressure returns to normal.

is there a (LAMMPS) question here anywhere?

axel.

>
> The pressure was increased in steps of 0.5 GPa.
> This is the script to increase the pressure:
>
> read_restart  restartnvtPRESSI.20000
> pair_style    vashishta/table 50000 0.2
> pair_coeff    * * BaS.vashishta Ba S
> timestep      0.0015  #1.5 fs
> thermo        100
> neighbor      2.8 bin
> neigh_modify  every 8 delay 0 check yes
> restart       10000  restartnptPRESS
> fix ensemble  all npt temp 300 300  0.1 iso PRESSI PRESS 0.8 drag 2.0
> run           10000
>
> It is followed by this:
>
> read_restart   restartnptPRESS.10000
> pair_style     vashishta/table 50000 0.2
> pair_coeff     * * BaS.vashishta Ba S
> timestep       0.0015  #1.5 fs
> thermo         200
> neighbor       2.8 bin
> neigh_modify   every 8 delay 0 check yes
> reset_timestep 0
> restart 20000  restartnvtPRESS
> fix ensemble   all nvt temp 300 300 0.2
> run            20000
> variable EN equal etotal/64000
> variable VN equal vol/64000
> variable VV0 equal vol/2092550
> variable P equal press/10000
> print "$P ${EN} ${VN} ${VV0}"  append EN_VN_VV0xP
>
> This is the output:
>
> P EN VN VV0
> 0.498214139358955 -4.67689463177975 32.6447608710287 0.998429999639595
> 1.00609741014146 -4.67648473762589 32.3362412413579 0.988994021383913
> 1.4966024340238 -4.6746881214147 32.0423089835447 0.98000419342279
> 1.99670055628326 -4.67275374841173 31.7467723854699 0.970965297206794
> 2.49344550866395 -4.66894193859381 31.4525314469158 0.961966028339878
> 2.98560163367427 -4.6646019792218 31.1504328713565 0.952726436054965
> 3.49976591713726 -4.65859003636735 30.8309554975335 0.942955318554943
> 3.99196104297199 -4.65127512178038 30.5018105031715 0.932888519845632
> 2.55781034810671     ***should be ~  4.5***   -4.64636945512424
> 29.9675446335295 0.916548162073014
> 5.04780562252056 -4.62419232422298 28.6805704512116 0.877186451400226
> 5.53334949714409 -4.61845496440199 28.4964903965933 0.871556419383992
> 6.09936839632278 -4.61258714695327 28.3038527127576 0.865664654902624
> 6.59943139470616 -4.60622700618407 28.1201803453888 0.860047091875885
> 7.09172245138479 -4.59988002438254 27.9447387010901 0.854681262990021
> 7.51737442008226 -4.59318803846492 27.7969135753358 0.850160076854313
> 7.96876802698317 -4.58736598507203 27.660466418695 0.84598688241451
> 8.46985762039892 -4.58087285861856 27.5201311405457 0.841694770970789
> 8.99764495769083 -4.57335941915581 27.3696805623856 0.837093286178433
> 9.54080552084891 -4.56532831238874 27.215101914371 0.832365545635585
> 10.0080632430966 -4.55739707829201 27.078257565823 0.828180203203113
> 10.4771320038008 -4.54977314026369 26.9551336719978 0.824414496670503
> 10.9504463447601 -4.54196627302447 26.8323356643082 0.820658757265406
> 11.5056615524498 -4.53347667234388 26.6982587670727 0.816558056482595
> 12.0129341982955 -4.52481688571975 26.5757242189331 0.812810374907036
> 12.5209653577354 -4.5160312699289 26.4505201209057 0.808981045966866
> 13.0274491660859 -4.50721985349281 26.3331360650914 0.805390890619506
> 13.5321324485457 -4.49815018656825 26.2191237476912 0.801903858857487
> 14.0312191401055 -4.48842291200214 26.1070046212487 0.798474729760301
> 14.5175704953188 -4.47977177005137 26.0004370179574 0.795215392296133
> 14.9822861897537 -4.47063864268584 25.8969510833331 0.792050306723049
> 15.4695401306553 -4.46111358869297 25.7922843036061 0.788849105364645
> 14.9435157758459 ***should be ~ 16.0*** -4.45561845048302 25.6834103775547
> 0.785519229726173
> 16.4158354690257 -4.42634437210097 25.3773616554825 0.776158823421604
> 17.1215011111232 -4.41367000502062 25.2510151154888 0.772294553244264
> 17.5444429543818 -4.40345351344091 25.1560723218056 0.769390757016824
> 13.59575376808  ***should be ~ 18.0***   -4.40286354134398 25.06217164511
> 0.766518833617855
> 18.4758510738129 -4.28351341218373 23.7545807307143 0.726526566517271
> 19.0138699041049 -4.27360461687247 23.664176878664 0.723761592427657
> 19.5159644902198 -4.26428152288067 23.583722825235 0.721300929877441
>
> Regards,
> Sandra
>
>
>
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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0

College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.