Optimal pair density functional for the description of nuclei with large neutron excess

Toward a universal description of pairing properties in nuclei far from stability, we extend the energy density functional by enriching the isovector density dependence in the particle-particle channel (pair density functional, pair-DF). We emphasize the necessity of both the linear and quadratic isovector density terms. The parameters are optimized by the Hartree-Fock-Bogoliubov calculation for 156 nuclei of mass number A=118 to 196 and asymmetry parameter (N-Z)/A<0.25. We clarify that the pair-DF should include the isovector density dependence in order to take into account in a consistent manner the effect of the isoscalar and isovector effective masses in the particle-hole channel. The different Skyrme forces can give the small difference in the pairing gaps toward the neutron drip line if the optimal pair-DF, consistent with the particle-hole channel, is employed.