Orbital optimization in quantum Monte Carlo applied on solids

The accuracy and efficiency of quantum Monte Carlo (QMC) methods can be improved directly by adapting the many body wavefunction employed in the importance sampling. Full wavefunction optimization introduced a decade ago enabled solving scientific challenges beyond chemical accuracy even with a very compact wavefunction ansatz. Recently algorithmic development paved the way for simulation with large electron counts although demonstrations were mostly limited to small molecular systems. When applying QMC in solids, using fixed orbitals calculated by density functional theory (DFT) together with variationally optimized Jastrow factors is still the most practical way. Recent study on bandgaps hints the necessity of improving single particle orbitals beyond Hartree-Fock or DFT. Thus, we enable orbital optimization schemes like rotating an extended set of orbitals and further directly optimizing orbital shapes with much more challenging amount of paramters. The improvement are benchmarked on a few selected solid state systems.