  We introduce a novel many body method which combines two powerful many body
techniques, viz., quantum Monte Carlo and coupled cluster theory. Coupled
cluster wave functions are introduced as importance functions in a Monte Carlo
method designed for the configuration interaction framework to provide rigorous
upper bounds to the ground state energy. We benchmark our method on the
homogeneous electron gas in momentum space. The importance function used is the
coupled cluster doubles wave function. We show that the computational resources
required in our method scale polynomially with system size. Our energy upper
bounds are in very good agreement with previous calculations of similar
accuracy, and they can be systematically improved by including higher order
excitations in the coupled cluster wave function.
