<div dir="ltr">Dear all, I have a question regarding the correct choice of system size. I guess this is also aiming at a more general understanding on how periodic DFT works. When is it important to expand the single unit cell to a supercell?? In my particular case I am studying a molecular crystal containing an organometallic fragment (a rhodium complex) embedded in cavities spanned by the counterion (the BArF4- borate anion). There are essentially 4 asymmetric units in the unit cell (4 cation/anion pairs) In another, but related, system there are only two of these asymmetric units involved. So how are the interactions between fragments at the boundary of the unit cell described? Since I am setting up the calculations as PERIODIC XYZ, am I getting away with just using the unit cell? Are all the interactions due to the presence of the neighboring cells in each direction automatically taken into account? In particular, here in this system each cation in the crystal is surrounded by 6 counterions (which form kind of an octahedral cavity), but of course in the unit cell each cation has only half (=3) anions in its vicinity. I suspect that I might have to use a supercell to describe the system properly, but what would be a good size? In the other case mentioned, there are only 2 asymmetric units present in the unit cell, and I wonder if I would introduce some sort of "size-error" into the calculations, which might make a comparison between the two different crystal systems difficult. I have attached the xyz for one system for reference. A second question, is it generally better to fully relax the complete cell or is a geometry optimization suffcient. Thanks for your help, much appreciated Tobias </div>