"fix npt" command can not control the pressure of copper

Dear professors,

I was trying to use “fix npt” command to control the pressure of copper, but I failed. The pressure fluctuated sharply. I have tried different ways to solve this problem, such as adjusting the Tdamp and Pdamp and enlarging the simulation time. But they did not work.

The following is the code and it is very simple, and I just want to increase the temperature of copper while keeping its pressure constant.

Best wishes

Xiaohui She

.................................code.............................................
units metal
boundary p p p
atom_style atomic

neighbor 0.5 bin
neigh_modify every 5 delay 0 check yes

variable x equal 200
lattice fcc 3.61

region box block 0 8 0 8 0 5
create_box 1 box
create_atoms 1 box

pair_style eam
pair_coeff * * Cu_u3.eam

fix 1 all nvt temp $x $x 1.0 drag 0.2

timestep 0.01
thermo 1000

run 50000

unfix 1
fix 1 all npt temp 200 2000 1.00 aniso 1.0 1.0 10 drag 0.2

dump 1 all atom 1000 copper.lammpstrj

run 100000

..................................partial simulation results............................................

Step Temp E_pair E_mol TotEng Press Volume
50000 196.95938 -4531.087 0 -4498.5249 8113.7969 15054.682
51000 212.54812 -4530.8335 0 -4495.6943 -647.74716 15152.443
52000 239.79922 -4530.683 0 -4491.0385 -918.93489 15159.277
53000 252.06569 -4530.5591 0 -4488.8866 665.57257 15143.768
54000 266.4961 -4530.7788 0 -4486.7207 172.93783 15150.472
55000 278.88155 -4530.6682 0 -4484.5624 -340.24584 15157.958
.................................................................................................................
140000 1828.3983 -4510.0372 0 -4207.7596 582.08084 15401.689
141000 1805.0237 -4512.4451 0 -4214.0319 82.086476 15397.733
142000 1842.5671 -4514.2404 0 -4209.6203 -1531.1856 15415.792
143000 1890.1837 -4513.6672 0 -4201.175 2064.9848 15382.988
144000 1874.4799 -4508.7358 0 -4198.8398 -4903.6359 15474.187
145000 1948.8826 -4507.772 0 -4185.5754 706.66335 15421.753
146000 1971.0648 -4509.0487 0 -4183.1849 494.21516 15423.88
147000 2000.8366 -4499.6758 0 -4168.89 -3282.1469 15490.847
148000 1933.1849 -4498.575 0 -4178.9736 271.0274 15451.011
149000 2034.7051 -4495.9594 0 -4159.5743 -1821.8857 15495.808
150000 2058.5012 -4493.2913 0 -4152.9723 4637.1614 15431.046

Dear professors,

I was trying to use "fix npt" command to control the pressure of copper, but I failed. The pressure fluctuated sharply. I have tried different ways to solve this problem, such as adjusting the Tdamp and Pdamp and enlarging the simulation time. But they did not work.

The following is the code and it is very simple, and I just want to increase the temperature of copper while keeping its pressure constant.

this topic has already been discussed very many times. you are
neglecting three important issues: 1) pressure fluctuates much more
than temperature for typical system sizes of MD simulations, 2) solids
are not very compressible, so fluctuations are even larger, and 3) the
size of fluctuations depends on the system size.

axel.