The convergence of the average dipole moment and the average electric field in the ACN crystal using the SCME potential is shown in Table in the Supporting Information. With the inclusion of moments up to hexadecapole, we are able to converge both the average dipole moment and field up to the third decimal place in agreement with previous studies on ice Lastly, we performed simulations of liquid acetonitrile in order to estimate its density at room temperature, and compare to the experiment value (see the S.I. section 8 for computational details). The minimum of the average potential energy was found to be 0.85 g/cm 3 , about 9.9% higher than to the experimental density of 0.78 g/cm 3 . This is consistent with the too-high density of the PBE0+MBD-calculated ↵-phase of the solid, as can be seen from table in the supporting information. As mentioned in the introduction, the a-axis of the crystal is mainly governed by dispersion. Given that the error is twice as large along this direction, it appears that the PBE0+MBD level of theory seems to overestimate the attractive dispersion interactions. The point charge model, which also only has a single interaction site for the methyl group, is inadequate in describing this behaviour. The correct behaviour can also be recovered by employing more elaborate potential functions with many interaction sites. However, we have shown that the right balance between a simpler, 3-site, non-electrostatic potential and the single-centered molecular multipole model can reduce the overall complexity of the description, while still predicting the correct behaviour of the ACN dimer. The simplicity and small number of interaction sites of our model is attractive due to reduced computational cost of evaluating energy and atomic forces, which is particularly important when it is coupled to DFT in QM/MM schemes. Evaluating electrostatic interactions between the QM and MM subsystems scales with the number of MM electrostatic interaction sites, so keeping this number manageable lowers the computational cost of the coupling. It is important to describe the electrostatics in the MM region accurately and consistently with the DFT region. Both solid and liquid phases have increased densities compared to experimental values. By identifying that the error is largest along the dispersion-dominated a-axis in the crystal, we show that PBE0+MBD@rsSCS must overestimate the dispersion interaction. It has been shown that the inclusion of atomic 3-body exchange and dispersion interactions as Axilrod-Teller-Muto (ATM) terms can significantly decrease the densities of organic liquids , and could thus be a future avenue for enhancement of our potential.