Most previous atomistic simulations of heterophase interfaceshave neglected misfit, the discrepancy between theinteratomic length scales parallel to the interface of the twophases. The obstacles to quantitative calculations of interfaceenergies in the presence of misfit are assessed. The most straightforward approach is to perform simulations for asupercell whose size is of the order of the cube of thesmallest common periodic length scale (essentially thecoincidence-site-lattice periodicity), which varies inversely with the misfit parameter. Such supercells typically contain atleast thousands of atoms. First-principles simulations arehighly accurate, but are feasible only for a few selectedheterophase interfaces with large misfit. Classical interatomicpotentials, on the other hand, are efficient numerically, buttheir accuracy has not been demonstrated in the context ofheterophase interface calculations. An approximate formulationof the interface energy is presented here which can beevaluated numerically by first-principles calculations for supercells of only moderate size. This formulation explores therelationship between the interface energies for coherent andsemi-coherent interfaces. A numerical application to aninterface between tetragonal TiAl and perovskite Ti3AlC ispresented.