Hi. I have a system in which the pressure pzz is different from pxx and pyy because the system is anisotropic. However, their sum is zero, and I am trying to do a run in which the ratios of the cell lengths along x, y, and z are held constant and the total pressure P = (pxx + pyy + pzz)/3 is held at zero. I thought that the following command
fix 1 all npt 0.4 0.4 1 xyz 0 0 10
would do the trick, but for some reason the ratios of Lx to Ly to Lz change (greatly) during the run.
Here are snippets of the log file
units lj
atom_style angle
boundary p p p
processors 4 4 1
neighbor 0.3 bin
neigh_modify every 1 delay 0 check yes
dump 1 all custom 100000 configs.holdiso tag x y z ix iy iz
read_restart heatup.res.10000
orthogonal box = (-41.1127 -42.0356 -5.19833) to (43.7383 42.8154 5.00473)
4 by 4 by 1 processor grid
70000 atoms
69800 bonds
69600 angles
2 = max # of 1-2 neighbors
2 = max # of special neighbors
reset_timestep 0
group ends type 2
400 atoms in group ends
group notends type 1
69600 atoms in group notends
bond_style fene
bond_coeff 1 30 1.5 1.0 1
special_bonds 0 1 1
angle_style cosine
angle_coeff 1 0.75
pair_style lj/cut 1.5
pair_modify shift yes
pair_coeff * * 1.0 1.0 1.5
fix 1 all npt 0.4 0.4 1 xyz 0 0 10
Resetting global state of Fix 1 Style npt from restart file info
timestep .01
thermo_style custom step temp epair ebond etotal press pxx pyy pzz lx ly lz vol
thermo 100
restart 500000 heatup.res
run 10000000
Memory usage per processor = 3.9225 Mbytes
Step Temp E_pair E_bond TotEng Press Pxx Pyy Pzz Lx Ly Lz Volume
0 0.3987103 -2.4078843 20.357496 18.838813 0.018431475 -0.13542447 -0.15594064 0.34665954 84.851011 84.851011 10.203057 73458.892
100 0.40108556 -2.4081637 20.355631 18.8401 -0.030417656 -0.25440208 -0.12787674 0.29102585 84.885421 84.885421 10.207537 73550.766
200 0.3995556 -2.4056779 20.355978 18.840843 -0.027694001 -0.20666648 -0.18260281 0.30618728 84.885885 84.885885 10.207936 73554.44
300 0.40283211 -2.4083831 20.354151 18.841343 0.0071930841 -0.18056975 -0.13478428 0.33693328 84.874065 84.874065 10.206857 73526.183
(snip)
9999800 0.40074939 -2.3910793 20.3509 18.890757 0.062947775 0.096553237 0.088241034 0.0040490548 27.80659 27.80659 95.77338 74052.595
9999900 0.40154465 -2.3907971 20.348669 18.891117 -0.0041623688 0.068006223 0.0082873043 -0.088780634 27.808561 27.808561 95.783381 74070.825
10000000 0.40053655 -2.3903391 20.349659 18.891075 -0.031163708 0.015608897 0.0042295935 -0.11332961 27.809911 27.809911 95.791244 74084.099
Loop time of 87932.2 on 16 procs for 10000000 steps with 70000 atoms
So, as you see, the initial Lx Ly Lz are: 84.851011 84.851011 10.203057
and the final Lx Ly Lz are: 27.809911 27.809911 95.791244
So the shape of the box is changing, which I do not want, and I did not think would happen with the "xyz" option.
Any clues as to what I did wrong and how to correct it?
Thanks,
Rob