# [lammps-users] The lengths of the box change so unusually in the tension.

Everyone:
First ,I am sorry to everyone,because of my mistake, a mail with the same title without a word is sended to the postbox.
The mechanical property of ionic crystal CeO2 is studied in my simulation. The model is constituted with 666 units. The PBC is used . After the model is created, it is equilibrated at npt ensemble. The input file is next:

units metal
atom_style charge
boundary p p p
neighbor 2.0 bin
neigh_modify delay 0 every 20
kspace_style ewald 1e-4
pair_style buck/coul/long 8.0
pair_coeff * ……
velocity all create 20 87287 dist gaussian
fix 1 all npt 20 20 0.1 aniso 0.0 0.0 0.0 0.0 0.0 0.0 0.1 drag 1.0
//the pressure along three directions are all 0//
thermo_style custom step temp lx ly lz pxx pyy etotal pzz
thermo 100
timestep 0.001
run 10000
*************
The log.file shwos :
lx ly lz
32.46 32.46 32.46
32.833253 32.833426 32.833692
32.876539 32.876378 32.878648
32.861608 32.85543 32.886574
32.846644 32.838933 32.990818
32.819266 32.818256 33.231891
……………………
34.056832 31.317084 36.376327
34.083711 31.290673 36.388623
33.709794 31.812917 35.882539
33.868381 31.589514 36.132908
34.169784 31.150683 36.52039
Question :
From the log.file, it shows that the lx and lz increase, the case of the ly is opposite. But I think ,because the pressure is controled by this command: aniso 0.0 0.0 0.0 0.0 0.0. It indicates that all pressures are controled with the same method. The length along three directions should be similar,for example the final lengths are:32.456 32.460 32.459. Second, because the C11 of the CeO2 is about 400Gpa, so the change of the length is so big.In my opinion, the change of the length should not exceed 5% of the origin length . I can’t understand it. Is there any mistake in my in file or something else ?Can anyone explain it for me? Thanks!

|

What are the corresponding Pxx Pyy Pzz pressures. If they
are averaging to 0.0, then the code is equilibrating
the box as you ask. If you want to tie the 3 box lengths
together, then you can use xyz instead of aniso. A 6x6x6
system is small. You might have better luck with a bigger
system.

Steve