  In a recent publication [S. Groth \textit{et al.}, PRB (2016)], we have shown
that the combination of two novel complementary quantum Monte Carlo approaches,
namely configuration path integral Monte Carlo (CPIMC) [T. Schoof \textit{et
al.}, PRL \textbf{115}, 130402 (2015)] and permutation blocking path integral
Monte Carlo (PB-PIMC) [T. Dornheim \textit{et al.}, NJP \textbf{17}, 073017
(2015)], allows for the accurate computation of thermodynamic properties of the
spin-polarized uniform electron gas (UEG) 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 non-trivial enhancements
that we describe in detail. We compare our new simulation results with recent
restricted path integral Monte Carlo data [E. Brown \textit{et al}., PRL
\textbf{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.
