[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


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?