Most chemical problems have more than 50 electrons. QMC is only exact for energies. We can't live with them, we can't live without them. After all, electrons are fermions. QMC isn't included in Gaussian 90.
Some problems are easier, some are harder.
Some work has been done. From time to time, I will give some references to work on these problems, or you can help by sending mail to David Ceperley .