Schrodinger equation error estimates

In order to maintain the wave function antisymmetry, the diffusion QMC is normally used within the fixed node approximation, i.e. the nodes are fixed by the initial trial wave function. Unfortunately, the location of nodes for the exact wave function is far from trivial to determine, although simple approximations such as HF can give quite reasonable estimates. The fixed node diffusion QMC thus determines the best wave function with the nodal structure of the initial trial wave function. If the trial wave function has the correct nodal structure, the QMC will provide the exact solution to the Schrodinger equation, including the electron correlation energy. It should be noted that the region near the nuclei contributes most to the statistical error in QMC methods, and in many apphcations the core electrons are therefore replaced by a pseudopotential. 

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