  The neutron-drip transition in the dense matter constituting the interior of
neutron stars generally refers to the appearance of unbound neutrons as the
matter density reaches some threshold density $\rho_\textrm{drip}$. This
transition has been mainly studied under the cold catalyzed matter hypothesis.
However, this assumption is unrealistic for accreting neutron stars. After
examining the physical processes that are thought to be allowed in both
accreting and nonaccreting neutron stars, suitable conditions for the onset of
neutron drip are derived and general analytical expressions for the neutron
drip density and pressure are obtained. Moreover, we show that the neutron-drip
transition occurs at lower density and pressure than those predicted within the
mean-nucleus approximation. This transition is studied numerically for various
initial composition of the ashes from X-ray bursts and superbursts using
microscopic nuclear mass models.
