Most solid-state electronic structure calculations are based on quantum electrons and classical nuclei. These calculations either omit quantum zeropoint motion and tunnelling, or estimate it in an extra step. Such quantum effects are especially significant for light nuclei, such as the proton or its analogue, mu(+). We propose a simple approach to including such quantum behaviour, in a form readily integrated with standard electronic structure calculations. This approach is demonstrated for a number of vacancy-containing defect complexes in diamond. Our results suggest that for the NHV(-) complex, quantum motion of the proton between three equivalent potential energy minima is sufficiently rapid to time-average measurements At X-band frequencies.