  We present analytic gradients and derivative couplings for the simplest
possible multireference configuration interaction method, CIS-1D, an electronic
structure ansatz that includes all single excitations and one lone double
excitation on top of a Hartree-Fock reference state. We show that the resulting
equations are numerically stable and require the evaluation of a similar number
of integrals as compared to standard CIS theory; one can easily differentiate
the required frontier orbitals ($\mathtt{h}$ and $\mathtt{l}$) with minimal
cost. The resulting algorithm has been implemented within the Q-Chem electronic
structure package and should be immediately useful for understanding
photochemistry with $S_{0}$-$S_{1}$ crossings.
