  The electron-excitation induced bond strength variations in metals have been
predicted from density-functional theory calculations and observed
experimentally, while the microscopic mechanism has yet to be elucidated. Here,
we present a minimal model that reproduces the phonon hardening and softening
for fcc- and bcc-structured metals as a result of the electron thermal
excitation. We explain why the phonon mode softens at the N point for
bcc-structured metals.
