  A quantum Monte Carlo method is presented for determining multi-determinantal
Jastrow-Slater wave functions for which the energy is stationary with respect
to the simultaneous optimization of orbitals and configuration interaction
coefficients. The approach is within the framework of the so-called energy
fluctuation potential method which minimizes the energy in an iterative fashion
based on Monte Carlo sampling and a fitting of the local energy fluctuations.
The optimization of the orbitals is combined with the optimization of the
configuration interaction coefficients through the use of additional single
excitations to a set of external orbitals. A new set of orbitals is then
obtained from the natural orbitals of this enlarged configuration interaction
expansion. For excited states, the approach is extended to treat the average of
several states within the same irreducible representation of the pointgroup of
the molecule. The relationship of our optimization method with the stochastic
reconfiguration technique by Sorella et al. is examined. Finally, the
performance of our approach is illustrated with the lowest states of ethene, in
particular with the difficult case of the singlet 1B_1u state.
