An initial HF calculation is required to construct Ĥλ , which reduces to the HF Hamiltonian at λ = 0 and the true physical Hamiltonian at λ = 1. Solving the eigenvalue problem H λ Ψ λ = E λ Ψ λ yields Ψ λ , which adiabatically connects the HF wavefunction Ψ 0 (a single Slater determinant) to the true physical wavefunction Ψ 1 .