Refinement of the energy expression for infinite fermion systems with central forces

A refinement is made on the approximate energy expression proposed previously by Takano and Yamada for infinite zero-temperature systems of spin-1/2 fermions interacting through two-body central forces which may depend on the two-body spin states. This refinement is accomplished through incorporation of a certain part of the three-body-cluster exchange terms into the energy expression. The Euler-Lagrange equation is derived from the refined energy expression and solved numerically for liquid ³He and for neutron matter. The energy per particle for liquid ³He is in better agreement with experimental data, while the effect of this refinement is relatively small for neutron matter. The three-body cluster terms not included in the refined energy expression are evaluated perturbatively and shown to be negligibly small for liquid ³He as well as for neutron matter with central forces.