  The scale and complexity of quantum system to which real-space quantum Monte
Carlo (QMC) can be applied in part depends on the representation and memory
usage of the trial wavefunction. B-splines, the computationally most efficient
basis set, can have memory requirements exceeding the capacity of a single
computational node. This situation has traditionally forced a difficult choice
of either using slow internode communication or a potentially less accurate but
smaller basis set such as Gaussians. Here, we introduce a hybrid representation
of the single particle orbitals that combine a localized atomic basis set
around atomic cores and B-splines in the interstitial regions to reduce the
memory usage while retaining high speed of evaluation and either retaining or
increasing overall accuracy. We present a benchmark calculation for NiO
demonstrating a superior accuracy while using only one eighth the memory
required for conventional B-splines. The hybrid orbital representation
therefore expands the overall range of systems that can be practically studied
with QMC.
