 One of the largest gains in accuracy over writing the wave function separably comes from the introduction of the so-called Jastrow factor.In this case the wave function is written as formula_14, where formula_15 is the distance between a pair of quantum particles and formula_16 is a variational function to be determined.With this factor, we can explicitly account for particle-particle correlation, but the many-body integral becomes unseparable, so Monte Carlo is the only way to evaluate it efficiently.