  Popular models of the phenomenological relaxation operators that are widely
used in the master equation formalism for open condensed-matter systems have
significant flaws ranging from limited applicability to violation of
fundamental physical principles. We propose a relatively simple universal model
of the relaxation operator which is free from these flaws, has a correct static
limit, correct direct-current limit in a uniform electric field, includes both
interband and intraband transitions, and is valid for an arbitrary dispersion
of quasiparticles in a solid. We use the proposed operator to generalize the
Lindhard formula and derive explicit expressions for the relaxation operator
for Dirac materials with an unconventional energy spectrum of quasiparticles,
such as graphene and Weyl semimetals. We compare the linear susceptibility
spectra for graphene obtained with different relaxation models and show that
the proposed relaxation operator leads to physically meaningful behavior of the
susceptibility at low frequencies whereas the existing models become completely
invalid.
