[gmx-users] freezing or constraining

[Alex]

Dear all,

In studying the solid surfaces, I have noticed that some people freeze or
constrain the slab's atoms to their initial or relaxed coordinates, I was
wondering if doing that is a proper approach? If it is, where the penalty
energy of being frozen or constrained goes?
I can understand that freezing the middle atomic layers to mimic the bulky
behaviors is acceptable to some extent not the whole slab!

Also, what is the difference between freezing and constraining in this
issue?

Thanks,
Alex