Large-N SU(N) Yang-Mills theories with milder topological freezing

We simulate 4d SU(N) pure-gauge theories at large N using a parallel tempering scheme that combines simulations with open and periodic boundary conditions, implementing the algorithm originally proposed by Martin Hasenbusch for 2d CPN-1 models. That allows to dramatically suppress the topological freezing suffered from standard local algorithms, reducing the autocorrelation time of Q(2) up to two orders of magnitude. Using this algorithm in combination with simulations at non-zero imaginary theta we are able to refine state-of-the-art results for the large-N behavior of the quartic coefficient of the theta-dependence of the vacuum energy b(2), reaching an accuracy comparable with that of the large-N limit of the topological susceptibility.