  Standard procedures for local crystal-structure optimisation involve numerous
energy and force calculations. It is common to calculate an energy-volume
curve, fitting an equation of state around the equilibrium cell volume. This is
a computationally intensive process, in particular for low-symmetry crystal
structures where each isochoric optimisation involves energy minimisation over
many degrees of freedom. Such procedures can be prohibitive for non-local
exchange-correlation functionals or other 'beyond' density functional theory
electronic structure techniques, particularly where analytical gradients are
not available. We present a simple approach for efficient optimisation of
crystal structures based on a known equation of state. The equilibrium volume
can be predicted from one single-point calculation, and refined with successive
calculations if required. The approach is validated for PbS, PbTe, ZnS and ZnTe
using nine density functionals, and applied to the quaternary semiconductor
Cu$_{2}$ZnSnS$_{4}$ and the magnetic metal-organic framework HKUST-1.
