  We present a simple and efficient wave function ansatz for the treatment of
excited charge-transfer states in real-space quantum Monte Carlo methods. Using
the recently-introduced variation-after-response method [J. Chem. Phys. 145,
081103 (2016)], this ansatz allows a crucial orbital optimization step to be
performed beyond a configuration interaction singles expansion, while only
requiring calculation of two Slater determinant objects. We demonstrate this
ansatz for the illustrative example of the stretched LiF molecule, for a range
of excited states of formaldehyde, and finally for the more challenging
ethylene-tetrafluoroethylene molecule.
