The wavefunction for two-neutron halo nuclei is constructed by taking a Hartree-Fock determinant for the core multiplied by a general variational wavefunction for two valence neutrons, and explicitly antisymmetrizing the product with an appropriate antisymmetrizer. The variational expression of energy with a general many-body Hamiltonian is derived for the purpose of minimizing with respect to the choice of . As an alternative, the lowest-order terms in energy can be evaluated by choosing as the solution of the presently familiar three-body model with the core-valence potential (non-local, in general), computed from chosen , and including the effect of the overlap (not equal to unity). The higher-order correction terms can also be computed term by term with the same . Numerical calculations are in progress.