  The deep crustal heating, associated with exothermal nuclear reactions, is
believed to be a key parameter for describing the thermal evolution of
accreting neutron stars. In this paper, we present the first thermodynamically
consistent calculations of the crustal heating for realistic compositions of
thermonuclear ashes. In contrast to previous studies based on the traditional
approach, we account for neutron hydrostatic/diffusion (nHD) equilibrium
condition imposed by superfluidity of neutrons in a major part of the inner
crust and rapid diffusion in the remaining part of the inner crust. We apply a
simplified reaction network to model nuclear evolution of various
multi-component thermonuclear burning ashes (superburst, KEPLER, and extreme
rp-process ashes) in the outer crust and calculate the deep crustal heating
energy release Q, parametrized by the pressure at the outer-inner crust
interface, P_{oi}. Using the general thermodynamic arguments, we set a lower
limit on Q, Q>0.13-0.2 MeV per baryon (an actual value depends on the ash
composition and the employed mass model).
