I have a problem regarding atoms lost. I equilibrated crystal at a certain temperature with periodic boundary condition. then I want to heat that crystal by applying fixed boundary condition in x and y and periodic in the z direction and I also used fix wall/reflect command so it only expands in the z direction. but while I start simulation atoms lost is showing. I am not able to figure it out why it is happening. please, can anyone tell me my mistakes in above procedure? I tried a lot to figure out.