Choosing the right system size - supercell

[Tobias Kraemer]

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 
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