  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 the mass number A=118-196 and the 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 the effect of the
isoscalar and isovector effective masses in the particle-hole channel
consistently. 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.
