where n i is the number of interactions of a given symmetry, and J i is their associated energy. It should be noted that in the final expression, the total number of parameters, J i , to be determined from fitting to energies calculated from DFT, is not equal to 31, but rather 23, since the parameters do not form a linearly independent basis-the simplest reduction being (considering the up-down orientation along the c-direction) that an equal number of head-to-head and tail-to-tail interactions of the molecules must be present due to the periodic boundary conditions. In practice, 8 parameters are eliminated using QR-decomposition of the original design matrix. This also means that the parameters cannot be interpreted directly as interaction strengths, as they will contain indirect information about other interaction types. Fig. shows an illustration of how one of the training structures is represented as molecular vectors of the different sites. We represent the molecular cations as vectors with directions along the 1 13 -direction and its symmetry equivalents for the first 2c Wyckoff site, and the 113 -direction at its equivalents for the second one. This gives directions in agreement with experiment and previous computations, whereas we will use arbitrary units for the magnitude of the dipole moments and only consider their directions throughout. the 4 1x1x1 cells are chosen as the 4 symmetry unique ones possible within that cell size, and the rest are chosen randomly.