[lammps-users] How to create a free surface on top and fixed at the bottom

Hello everyone,

I want to model a system with a free top surface, and fixed boundary atoms on the bottom face and with periodic boundary conditions along X and Z directions. I am new to LAMMPS, so would like to know how should Iset up this system. Should I specify the boundary command as:

boundary p s p

or

boundary p f p

Also, to use a combination of Buckingham and Coulomb interaction potentials should I use:

pair style buck/coul/long

Also, I am modeling a perovskite structure, is it possible to create this crystal structure using lattice command and if so then how? After generating the structure, is it possible to create vacancies at random sites in the system?

Thanks,

Milind

Comments below.

Steve

Hello everyone,

I want to model a system with a free top surface, and fixed boundary atoms on the bottom face and with periodic boundary conditions along X and Z directions. I am new to LAMMPS, so would like to know how should Iset up this system. Should I specify the boundary command as:

boundary p s p

or

boundary p f p

Either is fine but they are differnt. The doc page explains the difference.

Also, to use a combination of Buckingham and Coulomb interaction potentials should I use:

pair style buck/coul/long

Again, the doc page explains what it computes. You would need to use PPPM with any “long” pair
style.

Also, I am modeling a perovskite structure, is it possible to create this crystal structure using lattice command and if so then how? After generating the structure, is it possible to create vacancies at random sites in the system?

See the lattice command doc page for how create arbitrary lattices. The delete_atoms porosity command
can delete random atoms.

Thanks Steve,
If I specify: boundary p s p, then does it make free surfaces on either sides? How can I fix atoms on one of the faces (boundary atoms) and have only one surface to be a free surface. Could you please tell me specific commands on how to do this?

Thanks,
Milind


Thanks Steve,
If I specify: boundary p s p, then does it make free surfaces on either sides? How can I fix atoms on one of the faces

either sides of what? you cannot have one face periodic and the
opposite face not. same as you cannot be half-pregnant.


(boundary atoms) and have only one surface to be a free surface. Could you please tell me specific commands on how to do this?

study the examples, read the manual, search through the mailing list archives.
everything you are looking for is there. nobody will write your input for you.

cheers,
axel.

I think, I didn’t make it clear, I understand that you cannot have one face periodic and other not. What I’m trying to do is have PBC only along X & Z axes and turn off PBS along Y, and then in the Y direction, I want to freeze the atoms on the bottom face so that they don’t move while atoms on the top face can move. I’m looking for how to freeze atoms on surface.


I think, I didn’t make it clear, I understand that you cannot have one face periodic and other not. What I’m trying to do is have PBC only along X & Z axes and turn off PBS along Y, and then in the Y direction, I want to freeze the atoms on the bottom face so that they don’t move while atoms on the top face can move. I’m looking for how to freeze atoms on surface.

it has been explained only about a hundred times on this list.

you can simply do the time integration only on the atoms that you want to move,
or you can initialize the velocities of those “fixed” atoms to zero and then use
fix setforce with 0.0 0.0 0.0 on them as well and then time integrate all.

try searching the mailing list archives or just plain google
and you’ll see that this will turn up.

… and annoyed responses from people that expect
that people asking questions invest at least some
noticeable effort into researching the existing material.

axel.