Friedemann Schautz Instituut-Lorentz, Universiteit Leiden, Niels Bohrweg 2, Leiden, NL-2333 CA, The Netherlands

I will present a quantum Monte Carlo method for determining multi-determinantal Jastrow-Slater wave functions for which the energy is stationary with respect to simultaneous optimization of orbitals and configuration interaction coefficients.

The approach uses the framework of the so-called energy fluctuation potential (EFP) method which minimizes the energy in an iterative fashion based on Monte Carlo sampling and a fitting of local energy fluctuations. The optimization of orbitals is treated on the same footing as the optimization of the configuration interaction coefficients through the use of single excitations to a set of external orbitals (super-CI), which are then resummed via natural orbitals. The method is extended to excited states using the optimization of state-averaged energies.

I will also discuss the relationship of our approach with the stochastic renormalization technique by Sorella et al.. Finally, I will illustrate the performance of the new method with the notoriously difficult case of the 1¹B_1u state of ethene.