[lammps-users] query on defining lattice

LAMMPS LAMMPS Mailing List Mirror
1

Dear Christopher,

It depends on what crystal structure you are going to use in your
system. For example, in case of CsCl, it is having two basis in a unit
cell, for which the lattice command will be

lattice custom 5.0 a1 1.0 0.0 0.0 a2 0.0 1.0 0.0 a3 0.0 0.0 1.0
basis 0.0 0.0 0.0 basis 0.5 0.5 0.5

Similarly to large no of basis also you can write the lattice command,
but you should know the position of atoms in a unit cell.

Hope so it will help you

Goodluck

Vijay

Thank you, Sir. Now I understood what itype arg means. The 2 basis atom
in the unit cell are the same, so itype=1.

I have 1 query on defining basis atom using lattice command. If my
system is a simple cubic or space group = P 1, then I will use lattice
… custom … basis 0 0 0 . However, my complicated organic crystal
has a space group = P 1_bar or P -1, which implies that the second
basis is in an inversion position to the first basis. If I were to use
lattice with custom command, then I’ll define basis 0 0 0 for the first
basis, but what fraction should I use to define the position for the
second basis?

Thank you very much.

Regards,
Christopher

2

Dear Vijay,

Thanks for your reply. Forgot to mention, I’m simulating an organic crystal with triclinic structure and space group = P -1. I have no problem on defining the lattice vector a1 a2 a3, but I’ve problem on defining the second basis. P -1 space group implies that the are 2 basis in a unit cell: P means primitive and -1 means the crystal has an inversion symmetry. Hence, the second basis is at inversion position to the first basis. Since it is a primitive crystal, so the first basis is defined as basis 0 0 0, but I dont know how to define the position for the second basis. Can anyone please help me?
Thank you.

Regards,
Christopher

3

Define an a1,a2,a3 that enclose both the atoms
in your basis set. There are many possible choices
for a1,a2,a3. For LAMMPS the basis atoms have
to be 0 to 1 for all 3 coords, i.e. they have to be inside
the unit cell. But that should be possible, you just
might need a different choice of a1,a2,a3 then you are
using.

Steve