All atomic positions were relaxed in structure optimisations, which used a BFGS optimiser, imposing a maximal gradient of 5•10 -6 Ha a0 -1 and a maximal displacement between optimisation steps of 2.5•10 -5 a0 (a0 = 0.529177 Å) as convergence criteria. Calculations of the vibrational modes and IR intensities employed the finite displacement method, using a partial Hessian approach in which only displacements of the atoms of 5-FU and (where relevant) of framework protons and framework oxygen atoms participating in hydrogen bonds were considered. Structure visualisations were prepared using VESTA. The analysis of vibrational properties made use of MOLDEN, using a broadening with a full width at half maximum of 10 cm -1 for plots of IR spectra. All AIMD simulations were carried out for a temperature of 298 K in the NVT ensemble, using a Nosé-Hoover thermostat with a timestep of 0.5 fs and a time constant of 50 fs. Three types of tasks were tackled with AIMD simulations: