About the periodic boundary

Dear all,

In my initial configration files, there are two different crystal with a little bit different lattice constant, and my question is how to set the periodic boundary to satisfy those different lattice constant to mimic the real experimental situations: growing expital film on substrate?

Many thanks,

XJ

Dear all,

        In my initial configration files, there are two different crystal
with a little bit different lattice constant, and my question is how to set
the periodic boundary to satisfy those different lattice constant to mimic
the real experimental situations: growing expital film on substrate?

simple: pick your box dimensions in x and y so that both crystals will
fit into the box with an integer number of replications of the minimal
unit cell, you may have to do some rounding of the lattice constants
to keep your system reasonably small (and deliberately incur some
error).

example: diamond and silicon
the ratio of the two lattice constants (at 300K) is 0.6567362, which
you can (crudely) approximate as 2/3 or (more accurately) as 44/67.

so if you use the diamond lattice as base lattice to define the box,
you need to define the box in multiples of 3 (in x and y, in z you'll
probably have some vacuum separating the layers and thus you don't
need to pay as much attention.)

of course, if the ratio is not as commensurate, you need to make a
larger error in the lattice constants or generate a larger system as
in the example above with the 67 diamond units to match 44 silicon
units.

axel.