Dear all users,
I constructed an orthogonal bicrystal with periodic boundary conditions(BC) in the two
directions in the plane of the grain boundary (x and z directions), and non-periodic BC (f)
in the normal direction of grain boundary (y direction). Two free surfaces, respectively at the
top and bottom of the bicrystal along y direction, are created. Now I want to obtain the lowest
energy of the grain boundary located in the middle of the bicrystal at 0 K and with pressures
(Pxx, Pyy and Pzz) being 0.0 bar. In the case of a bicrystal with ppp BC, in order to obtain
the lowest energy of the grain boundary, we often minimize the energy of bicrystal with “minimize”
command, and then use “fix box/relax” coupled with “minimize” to perform minimization.
Here, I referred to this method and tried as follows:
Step 1: use “fix aveforce 0.0 0.0 0.0”to enforce the block of atoms at each surface to move as
a single block, and then use“minimize” to perform an energy minimization of the whole system.
Step 2: considering the non-periodic BC in y direction, I use “fix 1 all box/relax x 0.0 z 0.0 vmax 0.0001 ”
to relax the system, and then “minimize” it with “fix aveforce 0.0 0.0 0.0”.
Because the non-periodic condition in y direction does not allow this dimension (Ly) to change,
and the target pressure (Pyy = 0.0) to be specified during relaxation, i got fixed Ly and quite
large Pyy after Step 2. Meanwhile, Lx and Lz changes with time, and the final pressures of these
two directions are quite close to 0.0 bar. As the doc introduced, atoms will rescale to new positions
during relaxation and minimization. (1) So do atoms just rescale along x and z directions in Step 2?
(2) If yes, is that just why the Pxx and Pzz will relax to the target values while the Pyy can not?
If both assumptions are right, then i wonder is there a way that will allow atoms to rescale along y
direction and thus the Pyy can relax to 0.0 bar in Step 2? If it is impossible to find such a way,
does it just mean that the “fix box/relax” can’t be used to obtain the lowest energy of grain boundary
in the case of pfp bicrystal ?
I sincerely appreciate any suggestion. Thanks in advance.