  Discrete breathers are ubiquitous structures in nonlinear anharmonic models
ranging from the prototypical example of the Fermi-Pasta-Ulam model to
Klein-Gordon nonlinear lattices, among many others. We propose a general
criterion for the emergence of instabilities of discrete breathers analogous to
the well-established Vakhitov-Kolokolov criterion for solitary waves. The
criterion involves the change of monotonicity of the discrete breather's energy
as a function of the breather frequency. Our analysis suggests and numerical
results corroborate that breathers with increasing (decreasing)
energy-frequency dependence are generically unstable in soft (hard) nonlinear
potentials.
