The cohesion of solid thorium dioxide is calculated within the fully ionic description, calculating exactly, after generating the electronic wavefunctions of the ions, those parts of each inter-ionic potential that do not arise from electron correlation. The wavefunctions of the anions are modified by a potential representing the environment in the crystal. This gives rise to a substantial contribution to the lattice energy. Density-functional theory is used to evaluate the contributions from electron correlation. The dispersion term is evaluated separately. The lattice energy, lattice parameter and bulk compressibility calculated here are in excellent agreement with experiment supporting the ionic description of thorium dioxide.