  We review the role that first-principles calculations have played in
understanding the effects of substrate-imposed misfit strain on epitaxially
grown perovskite ferroelectric films. We do so by analyzing the case of
BaTiO$_3$, complementing our previous publications on this subject with
unpublished data to help explain in detail how these calculations are done. We
also review similar studies in the literature for other perovskite
ferroelectric-film materials.
