In the case of systems where the intention is only to perform VMC calculations, the above optimisation scheme has proved to be essential. When using the new u function from section , there is no approximate way to obtain the values of the variational parameters in the way that the original Fahy prescription allowed functions to be approximately calculated. Therefore some form of variance minimisation is required to produce the wavefunction. To achieve state of the art accuracy in these VMC calculations it is clear that significant effort is required to ensure the quality of the trial wavefunction before performing the calculation. As the accuracy of the final VMC calculation is completely controlled by the accuracy of trial wavefunction, it is necessary to perform the optimisation procedure over an ensemble of configurations of equivalent size to the number of moves in the final VMC calculation. If several re-generations of configurations are required, this leaves the wavefunction optimisation as the most computationally expensive part of performing a VMC calculation.