  The theory of epitaxial strain energy is extended beyond the harmonic
approximation to account for large film/substrate lattice mismatch. We find
that for fcc noble metals (i) directions <001> and <111> soften under tensile
biaxial strain (unlike zincblende semiconductors) while (ii) <110> and <201>
soften under compressive biaxial strain. Consequently, (iii) upon sufficient
compression <201> becomes the softest direction (lowest elastic energy), but
(iv) <110> is the hardest direction for large tensile strain. (v) The dramatic
softening of <001> in fcc noble metals upon biaxial tensile strain is caused by
small fcc/bcc energy differences for these materials. These results can be used
in selecting the substrate orientation for effective epitaxial growth of pure
elements and A/sub p/B/sub q/ superlattices, as well as to explain the shapes
of coherent precipitates in phase separating alloys.
