Separable representation of the nucleon-nucleon interaction in the 3S1-3D1 channel

A separable representation for the two-nucleon T-matrix in coupled channels has been obtained from a unitary pole expansion for the potential and the method has been tested with the Reid soft-core potential in the 3S,_3D1 channel. A 3-term expansion provides a T-matrix that is sufficiently accurate for a perturbation theory calculation of the three-nucleon bound state, while a IS-term expansion closely reproduces the exact T-matrix. In addition, a unitary pole approximation is presented that gives a better fit to the phase shifts and coupling parameter than that of Siebert et al. (1972).