Quantum Monte Carlo with orbital optimization applied on solids

The accuracy and efficiency of quantum Monte Carlo (QMC) methods can be improved directly by improving many body wavefunction. Full wavefunction optimization introduced a decade ago enabled solving scientific challenges beyond chemical accuracy. Recently algorithmic development pushed the computational efficiency of wavefunction derivatives and thus paved the way for simulation with large electron counts although demonstrations are mostly limited to 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 common practice. Recent study on bandgaps shows qualitative improvement by simply adding limited multi determinant expansion. This hints the necessity of improving single particle orbitals restricted by DFT. Thus, we enable orbital optimization schemes like mixing occupied and unoccupied orbitals or directly optimizing orbital shapes and study their strength and weakness on solid state systems.