Dear all,
The lammps documentation states that a NPT ensemble cannot be used
without periodic boundary conditions (PBCs). A naive question perhaps
... but why is this so?
I ask this because for any group of atoms that are to be simulated,
one would specify some positions and first do a "NPT relaxation",
because the positions assigned may not be consistent with the
empirical potential used.
For example, in the shear example which is part of lammps, one should
first relax the Ni FCC crystal and then carry out the shearing is my
belief. In the example, this is done using a NVE ensemble and the
temperature is rescaled to 300 K. The shearing is then carried out.
This does not ensure that the pressure in the system fluctuates around
zero as seen below:
''''''''''''''''''''''''''''' Output of ./lmp_serial < in.shear
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Step Temp E_pair E_mol TotEng Press Volume
0 300 -8317.4367 0 -8263.8067 -7103.5557 19539.346
25 220.34459 -8272.5701 0 -8233.1798 5170.6956 19539.346
50 300 -8237.7876 0 -8184.1576 13746.642 19678.792
75 292.67269 -8229.8093 0 -8177.4891 14324.984 19737.233
100 300 -8245.8191 0 -8192.189 9038.4055 19770.872
Loop time of 0.871972 on 1 procs for 100 steps with 1912 atoms
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
In principle one would do the equilibration for many more steps using
a NPT ensemble and then carry out the shear.
However lammps does not allow fix NPT without periodic boundary conditions.
Is there something in the documentation I am missing OR is my recipe
for shear wrong?
With Best Regards,
Manoj Warrier
Institute for plasma Research, BHAT, Gandhinagar,
Gujarat, India - 382425
http://www.ipr.res.in/~manoj