Efficient localized basis set for quantum Monte Carlo calculations on condensed matter

We present an efficient scheme for representing many-body wave functions in quantum Monte Carlo (QMC) calculations. The scheme is based on B splines (blip functions), which consist of localized cubic splines centered on the points of a regular grid. We show that blip functions are unbiased, systematically improvable, and conveniently obtained from any standard plane-wave density functional theory (PW-DFT) code, and therefore provide a convenient and natural interface between PW-DFT and QMC calculations. We present tests on a 16-atom system of Si in the beta-tin structure, and on 2- and 8-atom systems of MgO in the NaCl structure. We show that already with such small systems the speed-up of blip functions with respect to plane waves is between one and two order of magnitudes, without compromising the accuracy.