A recently developed method for treating spin-forbidden electronic transitions within the Breit–Pauli approximation is shown to provide a means for characterizing such processes when the transitions derive their intensity by coupling to states embedded in a continuum. The success of this approach is attributable principally to the need to specify only the configuration state function space, rather than its spectrum relative to Ĥ0, in order to obtain the first order perturbation contribution (Ψ1I) to the wave function. Here Ĥ0 is the nonrelativistic Born–Oppenheimer Hamiltonian and Ψ1I is the solution of (Ĥ0−E0I) Ψ1I=−ĤsoΨ0I where Ĥso is the full microscopic spin–orbit portion of the Breit–Pauli interaction. A method for improving the molecular orbital basis used to describe Ψ1I based on the iterative natural orbital (INO) procedure is introduced. The a 1Δ→X 3Σ− transition in CH− was considered. Using the INO procedure, it was found that the optimum orbital space for describing Ψ1I includes a molecular orbital with character intermediate between the compact valence orbital and the diffuse orbital obtained from two alternative MCSCF procedures. Equivalent INO orbitals were obtained from these two distinctly different starting points. Using the INO orbital set, a total radiative rate for the ground vibrational level of the a 1Δ state of 0.163 s−1 was obtained which gives a lifetime, τ=6.14(±1.2) s. This value is in excellent agreement with the experimental value τ=5.9(+0.8, −0.6) s reported by Okumura et al.