# How can I scan the whole PES of a bulk structure?

I am not very familiar with scanning a PES for a bulk structure, and I'd be very appreciative for some suggestions.

• Welcome to our community! – Camps Jun 30 '20 at 17:59
• I think that as the bulk is periodic, you cannot scan the PES. All the atom positions are "forced" due to symmetry operations. – Camps Jun 30 '20 at 17:59
• Welcome to the site! If you haven't yet, take the tour and visit the help center. You will probably need a bit more detail to get a good answer. For example 1. what sort of compound are you looking at? 2. what modeling software are you trying to use? 3. are you just trying to map the PES or are you interested in some specific features (e.g. a particular transition state, the location of minima)?. These details will make it easier for someone to address your particular problem and will make the question more useful to future users. – Tyberius Jun 30 '20 at 17:59
• It is worth to limit what area of PES you want to scan: the whole PES would include all kind of geometries, even gas or liquid. What you are generally looking for is PES around equilibrium, or along a sub space, eg reaction path. All of these are extremely high dimensional spaces, so even there you generally use some approximation how to reduce dimensions. – Greg Jul 1 '20 at 6:46

The potential energy surface (PES) is a 3N-dimensional function for a bulk system containing N atoms (in reality 3N-3 to account for the trivial translational degrees of freedom). For a bulk structure, N typically represents the number of atoms in a simualtion cell with periodic boundary conditions, which is of the order of $$10^2$$-$$10^3$$, so the function is of very high dimension. The number $$10^2$$-$$10^3$$ comes from a typical DFT calculation; if you use cheaper methods (e.g. force fields) then that number can be larger, or if the method is more expensive (e.g. quantum chemistry) then the number is somewhat smaller. In any case, you are sampling a high dimensional function, which is an extremely hard problem.
• +1. Another good one from ProfM. As a molecular person, I would say the PES is 3N-6 dimensional or 3N-5 for linear molecules. So for the diatomic molecule H$_2$, we have 3N-5 = 3(2)-5 = 1 dimension, which is the internuclear distance R, between the two H atoms. Since the question is about bulk structures maybe your PES is typically 3N dimensional. Sorry if my comment is very naive! – Nike Dattani Jun 30 '20 at 22:56