As the neutron stars undergo accretion, the density in the crust increases, passing the electron capture threshold. As the electron capture threshold ( g cm) is exceeded, it allows for the formation of light nuclei from the process of double electron capture (), forming the light neon nuclei and free neutrons, which further increases the density of the crust.  As the density increases, the crystal lattices of neutron-rich nuclei are forced closer together due to gravitational collapse of accreting material, and at a point where the nuclei are pushed so close together that their zero-point oscillations allow them to break through the Coulomb barrier, fusion occurs. While the main site of pycnonuclear fusion within neutron stars is the inner crust, pycnonuclear reactions between light nuclei can occur even in the plasma ocean. Since the core of neutron stars was approximated to be  g cm, at such extreme densities, pycnonuclear reactions play a large role as demonstrated by Haensel & Zdunik, who showed that at densities of  g cm, they serve as a major heat source. In the fusion processes of the inner crust, the burning of neutron-rich nuclei () releases a lot of heat, allowing pycnonuclear fusion to perform as a major energy source, possibly even acting as an energy basin for gamma-ray bursts.
Further studies have established that most magnetars are found at densities of g cm, indicating that pycnonuclear reactions along with subsequent electron capture reactions could serve as major heat sources.