Dear Yijin,
Please keep the mailing list in cc.
Dear Vasily,
I replace N with N-1, which is 4-1 = 3, then pressure finally match
very well.
So I wanna make sure what N refer to if it is not the number of
particles?"Note that the N in the first formula above is really
degrees-of-freedom divided by d = dimensionality, where the DOF value
is calcluated by the temperature compute. See the various compute
temperature styles for details. "Sentence in bold is cited from
LAMMPS Molecular Dynamics Simulator.DOF value for an one-atom molecule is 3, isn't it? Then total DOF of
the 4 atom system should be 3*4=12, since there are some constrains to
atoms motion, I guess there are 3, so finally N should equals to (12 -
3)/3 = 3; Does this explanation make sense? If it does, I still have
troubles with counting the number of constrains. To my knowledge,
velocity distribution is one kind of constrain.
3 DOFs are excluded from temperature calculation because they correspond
to motion of a system as a whole (other DOFs are "thermal" motion). That
was discussed in the mailing list before.
Interestingly, if I increase the number of atoms from 4 to 4000 by
changing these four lines
create_atoms 1 single 0 -0.889687 -0.889687 units box
create_atoms 1 single -0.889687 0 -0.889687 units box
create_atoms 1 single -0.889687 -0.889687 0 units box
create_atoms 1 single 0 0 0 units box
to
create_atoms 1 boxThe pressure comes to be weird again.
You could try looking into the code and find out how pressure
calculation is implemented.
Best regards,
Vasily