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


From: Sandra Cristina Costa Prado <sandra.prado01@...6996...>
Date: Mon, 17 Jul 2017 13:28:26 +0000

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.

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