Negative sign problem in continuous-time quantum Monte Carlo: Optimal choice of single-particle basis for impurity problems

The negative sign problem in quantum Monte Carlo (QMC) simulations of cluster impurity problems is the major bottleneck in cluster dynamical mean-field calculations. In this paper, we systematically investigate the dependence of the sign problem on the single-particle basis. We explore both the hybridization-expansion and interaction-expansion variants of continuous-time QMC simulations for three-site and four-site impurity models with baths that are diagonal in the orbital degrees of freedom. We find that the sign problem in these models can be substantially reduced by using a nontrivial single-particle basis. Such bases can be generated by diagonalizing a subset of the intracluster hoppings.