# [lammps-users] press computation about rigid

In the simulation, tension behavior of copper is simulated. The process is carried out
by the rigid command. In the next file, the boundary atoms are treated as rigid. The input file
is next:
units metal
atom_style atomic
boundary s p s
lattice fcc 3.62
region box block 0 20 0 8 0 20
create_box 1 box
create_atoms 1 box
mass 1 63.54
region bottom block INF INF INF INF INF 1
region top block INF INF INF INF 19 INF
region boundary union 2 top bottom
group top region top
group bottom region bottom
group boundary region boundary
group mobile subtract all boundary
pair_style eam
pair_coeff * * Cu_u3.eam
neighbor 0.3 bin
neigh_modify every 20 delay 0 check no
velocity mobile create 20 87287 dist gaussian
fix rigid1 top rigid single force 1 off off off torque 1 off off off
fix rigid2 bottom rigid single force 1 off off off torque 1 off off off
fix temp mobile nvt temp 20 20 0.05 drag 1.0
compute peratom all stress/atom
compute pzztop top reduce sum c_peratom[3]
compute pzzbottom bottom reduce sum c_peratom[3]
compute pzzmobile mobile reduce sum c_peratom[3]
compute pzz all reduce sum c_peratom[3]
variable pzztop equal -c_pzztop/vol
variable pzzbottom equal -c_pzzbottom/vol
variable pzzmobile equal -c_pzzmobile/vol
variable pzzall equal -c_pzzall/vol
thermo 50
thermo_style custom step temp pzz v_pzzall v_pzztop v_pzzbottom v_mobile

velocity top set 0.0 0.0 0.3 units box
velocity bottom set 0.0 0.0 -0.3 units box
run 20000
In the simulations, in order to compare the pressure of different groups, the volume in the
compute commands are all the same: vol.
In the file, pzztop means the pzz of group top, pzzbottom means the pzz of group bottom, similar for
pzzmobile and pzzall.
After simulation, the results are next:
Step Lz Pzz pzzall pzztop pzzbotto pzzmobil
0 72.4002 300.54575 300.54575 10148.067 -668.72384 -9178.7974
50 72.4242 9484.6457 9484.6457 19004.588 -1250.6594 -8269.2832
100 72.4602 11038.081 11038.081 20583.761 -1227.4254 -8318.255
150 72.4842 7876.338 7876.338 17986.07 -1104.7665 -9004.9656
200 72.5202 5990.2187 6020.1638 16474.871 -1169.6901 -9285.0171
250 72.5442 5383.0511 5383.0511 16050.751 -1090.4566 -9577.2428
300 72.5802 5458.3817 5490.693 16645.386 -1005.8974 -10148.795
350 72.6042 4846.5032 4846.5032 16312.414 -1055.4686 -10410.442
400 72.6402 3366.3345 3367.7899 15225.957 -1104.3081 -10753.859
450 72.6642 2910.9124 2910.9124 15176.167 -1017.6607 -11247.594
500 72.7002 2159.8354 2256.2399 14604.4 -883.7485 -11464.412
550 72.7242 -291.80089 -291.80089 12457.853 -857.38513 -11892.269
600 72.7602 -845.65559 -845.65559 12241.596 -869.07617 -12218.176
650 72.7842 426.20345 426.20345 13782.483 -813.20975 -12543.07
700 72.8202 -387.92133 -387.92133 13293.178 -807.48349 -12873.616
750 72.8442 -1207.4904 -1207.4904 12802.03 -862.9464 -13146.574
800 72.8802 -262.58689 -262.58689 14033.495 -830.63116 -13465.451
850 72.9042 -674.47452 -674.47452 13855.829 -807.94051 -13722.363
900 72.9402 -1952.1566 -1952.1566 12771.765 -820.97177 -13902.949
950 72.9642 -1122.3727 -1122.3727 13857.67 -782.03704 -14198.006
1000 73.0002 -1452.7076 -1452.7076 10826.194 -811.39702 -11467.505
1050 73.0242 -3753.1102 -3753.1102 11682.813 -796.88845 -14639.034
1100 73.0602 -3890.6928 -3890.6928 11733.521 -762.12147 -14862.092
1150 73.0842 -2778.9873 -2778.9873 12999.258 -710.5268 -15067.719
1200 73.1202 -3647.9183 -3647.9183 12307.884 -707.66117 -15248.141
1250 73.1442 -4526.9701 -4526.9701 11694.895 -700.72124 -15521.144
1300 73.1802 -4712.6583 -4712.6583 11759.756 -710.19308 -15762.221
1350 73.2042 -5030.1176 -5030.1176 11545.351 -758.7049 -15816.763
1400 73.2402 -5063.3472 -5063.3472 11595.993 -722.26536 -15937.074
1450 73.2642 -4020.3468 -4020.3468 12703.63 -625.0809 -16098.895
1500 73.3002 -2574.5591 -2675.8328 14296.032 -800.04358 -16171.821
1550 73.3242 -595.89933 -595.89933 16654.753 -1098.0991 -16152.553
1600 73.3602 3443.6938 3803.7643 20895.26 -1236.6599 -15854.836
1650 73.3842 7427.392 7427.392 24922.807 -1371.7766 -16123.639
1700 73.4202 7258.6569 7236.0921 24937.414 -1528.6188 -16172.703
1750 73.4442 4001.4806 4001.4806 21776.835 -1395.2408 -16380.113
1800 73.4802 1094.4812 1094.4812 18844.745 -1051.934 -16698.329
I am puzzled with the results. From the resluts, the lz increases continuously with the time increases.
In theory, pzz should increase, however, from the results above, pzz is equal to pzzall, and they nearly does not increase. In addition, the pzztop should be equal to pzzbottom in my opionion (for group top and bottom, in virial formula, only ke term exits for them, so the pzz for them should be small), actually, these two variables are so different in the results. Furthermore, the absolute value of pzzmobile increases continuously. I am so puzzled with the results, should anyone can tell me something about the press computation when the rigid body is introduced in the model? Thanks! The attachment is Cu_u3.eam file.

Cu_u3.eam (35.6 KB)