Hello,

I have a question on the periodic boundary conditions. I was under the

impression that when using these BCs any size box should behave as a bulk

material. One of the criteria used to determine when a crystal has melted is

the Lindemann criterion, e.g melting occurs when the RMSD of atoms exceed

10% of the lattice constant. However, I have noticed that when the box is

defined smaller than 40 x 40 x 20 nm the RMSD exceeds 10% of the lattice

constant a temperatures much lower than the bulk melting point. Why would

this happen if the simulated material is a bulk and the periodic boundary

conditions make it such that end effects are negligible? I was expecting

that one could define a box of any size with periodic boundary conditions

and get the same results.

Also, I have some problems with the timestep used for the EAM potential.

When I define a free surface the recommended value of 0.001 ps works fine.

However, the same timestep for the case with periodic boundaries on all

sides produces large oscillations on the RMSD when the temperature

increases. I have found that 0.005 ps eliminates these oscillations, but I

would like to know why this is the case.

Thanks

Jaime Sanchez