compression test using fix wall/lj126

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1

Dear Lammps users,
I want to perform a compression test in z-direction, but I want to have non-periodic bc in z-direction. That’s why I decided to use fix wall/lj126 to compress the system. I want the simulation box to change in x and y direction, so I couple it with fix npt in x and y direction. But unlike what I expected after a few compression steps it gives the following error.

The box size in x and y direction only fluctuates, and the change is not comparable with the compression. I even tried using NVT and it again did not change box size in x and y directions. Attachment is the input file, the log file for both using NPT(in x and y direction) and NVT. I was wondering how should I solve this issue to observe system size change in x and y directions.

Best,
Sepideh

Step KinEng PotEng Press Lx Ly Lz Pxx Pyy Pzz
0 7.6392746 -650.50999 -9413.5585 12.18 12.18 28.42 -11967.777 -11960.251 -4312.6478
1000 7.820194 -643.40206 6237.5802 12.085471 12.127597 28.42 4706.2858 3346.1076 10660.347
2000 7.7440042 -643.04421 -803.09661 12.139826 12.170599 28.42 -2045.5918 -2920.4301 2556.732
3000 7.8925054 -643.20869 786.7776 12.11608 12.161241 28.42 378.32171 -1148.0517 3130.0628
4000 7.8157654 -643.08568 3413.9904 12.079667 12.105849 28.42 2538.9482 2455.7125 5247.3106
5000 7.6975426 -643.02383 -2151.9124 12.118654 12.159658 28.42 -3151.385 -3679.3052 374.95305
6000 8.0299079 -643.30911 2671.2928 12.108236 12.071537 28.42 1328.6455 2643.7503 4041.4827
7000 7.7985099 -643.02726 -2978.9436 12.143986 12.122379 28.42 -3934.7183 -4117.4212 -884.69133
8000 7.746233 -643.06364 1439.1423 12.087154 12.071605 28.42 831.18484 1956.5382 1529.7038
9000 7.6897774 -642.96316 36.898684 12.098967 12.07307 28.42 58.872743 244.63712 -192.81381
10000 7.9710423 -643.26397 -2003.4959 12.109156 12.066954 28.42 -2424.4851 -1087.6126 -2498.39
11000 7.9182251 -643.21072 -1137.2981 12.087996 12.061811 28.42 -184.46689 -744.8084 -2482.619
12000 7.4211633 -642.63787 -582.1693 12.080111 12.053602 28.42 -246.20735 100.06814 -1600.3687
13000 7.3892051 -642.6271 697.17116 12.082088 12.059888 28.42 1411.9426 430.38346 249.18741
14000 7.061011 -642.39836 -1682.3495 12.093979 12.065977 28.42 -1559.2894 -1380.186 -2107.573
ERROR: Lost atoms: original 198 current 180 (src/thermo.cpp:441)
Last command: run ${step}

compression_step.lammpstrj (122 KB)

test.in (2.56 KB)

log-nvt.log (7.17 KB)

log-npt.log (7.17 KB)

2

A few things. Your box will not expand in the X,Y direction when you are using fix NVT because there is no driving force for it to do so. Saying you are doing NVT and forcing the wall to move is not an appropriate set up because then you are no longer conserving volume. How large of a deformation do you expect? What type of a material is it? What is the rate dependence? Another thing to consider is physically think about what you are doing and what the rate is. I don’t think your error is all that unexpected.

Best,
Dylan