  We study the effect of physics input on thermal evolution of neutron stars in
soft X-ray transients (SXTs). In particular, we consider different modern
models of the sources of deep crustal heating during accretion episodes and the
effects brought about by impurities embedded in the crust during its formation.
We simulate thermal structure and evolution of episodically accreting neutron
stars under different assumptions on the crust composition and on the
distribution of heat sources and impurities. For the nonaccreted crust, we
consider the nuclear charge fluctuations that arise at crust formation. For the
accreted crust, we compare different theoretical models of composition and
internal heating. We also compare results of numerical simulations with
observations of the crust cooling in SXT MXB 1659-29. We found that the
nonaccreted part of the inner crust of a neutron star can have a layered
structure, with almost pure crystalline layers interchanging with layers
composed of mixtures of different nuclei. The latter layers have relatively low
thermal conductivities, which affects thermal evolution of the transients. The
impurity distribution in the crust strongly depends on the models of the dense
matter and the crust formation scenario. The shallow heating that is needed to
reach agreement between the theory and observations depends on characteristics
of the crust and envelope.
