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


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Tue, 18 Jul 2017 13:32:17 -0400



On Tue, Jul 18, 2017 at 1:26 PM, Sandra Cristina Costa Prado <sandra.prado01@...6996...> wrote:


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)?


​have you discussed this with your adviser/supervisor? this isn't really a question about LAMMPS, but about your research. since this is a *LAMMPS* users mailing list, the preferred topic are discussions about how to make LAMMPS do certain things or reports when LAMMPS does not do what the documentation says it would do and so on.

​have you done any research on the literature to see, if somebody else has observed that the processes you are looking into are activated, i.e. may not be spontaneous and immediate? you may to cast a wider net and look for similar systems or compounds.

axel.

 


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@...396...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@...6999...br> 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
The Homepage of Axel Kohlmeyer

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



--
Dr. Axel Kohlmeyer  akohlmey@...92......  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.