Computations incorporating the three-phonon interaction process are presented of phonon spectral functions, yielding frequency and lifetime dispersion curves, for the bcc solid phase of both ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$. Comparison of the compressibilities obtained from ground-state energy and elastic constants as functions of molar volume quantify the effects of certain analytical approximations. A model calculation shows the sensitivity of elastic anisotropy and long-wavelength dispersions to the precise form of the three-phonon vertex.

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