However, it is known that the crystalline electrostatic potential converges very slowly in real space, and finite supercell expansions of lattice charges do not correctly approach the exact potential. This is because atomic charges within a unit cell are only balanced by their periodic neighbouring charges, and so the surfaces of a finite slab remain charged, generating a static electric field that deviates from that in the infinite crystal. This is clearly observed in the electrostatic potential map, where there is an obvious lack of symmetry for the finite supercell expansions compared to the exact result for an infinite crystal (Figure ), and also in the corresponding equilibrium electronic CF splitting for 1 calculated with the different electrostatic potentials (Figure ). We note that we checked larger supercells up to a 9×9×9 expansion and still did not observe convergence (Figure ). The electrostatic potential can be converged very quickly in reciprocal space using Ewald summation, 66 but this method is not available in OpenMolcas. Luckily, there are two methods that can be employed to address this issue: the first uses an extra set of charges external to the crystalline slab as fitting parameters to replicate the exact infinite crystalline potential, and the second places an approximately spherical supercell in a perfect conductor to screen the charges directly, thus approximating the infinite crystalline potential very closely. The first method is not compatible with our analytic "one-shot" LVC method for obtaining the spin-phonon coupling coefficients, and we have therefore used the conductor screening method which allows us to obtain spin-phonon coupling coefficients corrected for the infinite crystalline electrostatic potential (see Methods). To illustrate the effect that the finite supercell and thus inaccurate electrostatic potentials, have on the spin dynamics, we have computed single-phonon (Orbach) and twophonon (Raman) rates using fixed phonon linewidths for four finite supercell expansions (Approach 1, see Methods). We observe that the rates calculated using finite supercell expansions oscillate above and below the experimental rates for even and odd supercells, respectively, and do not converge (Figure ). Using even and odd supercell expansions yields faster and slower rates, respectively, which is consistent with smaller (larger) CF splitting for the former (latter). The small electric potential developed over a finite supercell thus has a nonnegligible impact on the spin dynamics of molecular crystals, affecting the magnetic relaxation rates by ca. one order of magnitude. We therefore adopt a method that allows us to include the infinite crystalline electrostatic potential (Approach 2, see Methods), but we still compare the different q-point grids to assess the impact of Brillouin Zone integration for the phonons.