How close can we get to the lowest energy molecular structures just by maximizing packing density? Let’s compare the structures Andy obtained from Materials Studio by minimising some force-field potential
with the structures I got from maximizing packing density: https://milotorda.net/getting-loads-of-sufficiently-dense-structures/ .
To get a clearer view, we took Andy’s set, created a neighborhood around the packing coefficient, divided the interval into 
bins, and in each bin, selected the lowest energy structure. Then, we calculated the energies of these structures using VASP’s DFT (PBE) and computed the packing density via https://milotorda.net/exercise-in-monte-carlo-density-computation/
Next, we selected around structures generated here https://milotorda.net/getting-loads-of-sufficiently-dense-structures/ and computed their energies using VASP’s DFT, just like we did for Andy’s set.
The energy difference between the lowest energy structure in Andy’s set and mine is 
. However, in Andy’s structures, the van der Waals spheres of different anthracene molecules overlap. If we filter out the structures with overlaps,
then the energy difference between the lowest energy structures in both sets is 
.
We then continued to further optimise the energy of all structures using VASP’s implementation of the conjugate gradient method. Here, the molecules are no longer rigid, meaning the bond lengths of the molecules in each structure change.
To be honest, I didn’t mention that we performed DFT calculations for a broader density spectrum from Andy’s CSP set. At the time of the DFT calculations, I didn’t have density estimates for all configurations, so I used the packing coefficient given by Materials Studio to guide the selection of structures. The density estimates I used are slightly different. Here is the full picture,
which brings up the question of how far from the highest density structure (in the density spectrum) do we have to look for the lowest energy structure.