Ab initioquantum Monte Carlo simulations of the uniform electron gas without fixed nodes: The unpolarized case

In a recent publication S. Groth et al., Phys. Rev. B 93, 085102 (2016), we have shown that the combination of two complementary quantum Monte Carlo approaches, namely configuration path integral Monte Carlo T. Schoof et al., Phys. Rev. Lett. 115, 130402 (2015) and permutation blocking path integral Monte Carlo T. Dornheim et al., New J. Phys. 17, 073017 (2015), allows for the accurate computation of thermodynamic properties of the spin-polarized uniform electron gas over a wide range of temperatures and densities without the fixed-node approximation. In the present work, we extend this concept to the unpolarized case, which requires nontrivial enhancements that we describe in detail. We compare our simulation results with recent restricted path integral Monte Carlo data E. W. Brown et al., Phys. Rev. Lett. 110, 146405 (2013) for different energy contributions and pair distribution functions and find, for the exchange correlation energy, overall better agreement than for the spin-polarized case, while the separate kinetic and potential contributions substantially deviate.