# Lattice constant of an FCC crystal from molecular dynamics simulation

So, I have a system of n number of molecules inside a box which is supposed to be solid. I performed the md simulation of the system using stochastic integrator in a box. I know the lattice of the crystal, and thus the theoretical, and experimental lattice constant. Is there anyway to compute the lattice parameter of such system after you run an unbiased simulation for a while, and compute the lattice parameter from the information related to the box? Thanks.

• I think that is possible, using radial distribution function or performing a Fourier transform of the atomic positions to identify the atomic/molecular positions that correspond to lattice sites, then measure the distances between neighboring lattice sites along the lattice vectors and average them over multiple lattice vectors to obtain an estimate of the lattice parameter. I am supposing that you can can extract the position coordinates of the atoms from the trajectory and you have applied periodic boundary conditions Commented Dec 14, 2023 at 9:05
• What's wrong with running a NPT ensemble and averaging the lattice parameters over time? Commented Dec 14, 2023 at 23:16
• Hi Ian, but how do you get the lattice constant, the one I am looking for- from the size of the box??? Commented Dec 15, 2023 at 5:40
• You have the lattice vectors, i.e. the size of the box. You know how many unit cells this is supposed to be. You should be able to work out the shape of an individual unit cell from that. Commented Dec 15, 2023 at 8:26
• I think Ian's answer is correct, as long as there is indeed a single grain in the cell. Depending on how large n is and how the structure was initialized, the simulation could in principle also contain grain boundaries. In the radial distribution function that may give rise to side peaks that could be ignored, while looking at density of molecules in the simulation box includes the boundary in the average. Commented Dec 17, 2023 at 23:00