Ground state structure of BNB using fixed-node diffusion Monte Carlo

► What is the nature of ground state geometry of BNB: symmetric or asymmetric? ► Convergence of MR-CCSD (T) with respect to number of determinants and basis size. ► QMC energy gap is lower than that of the CCSD (T). ► BNB has a highly floppy quasi-symmetric configuration. ► Quantum Monte Carlo methods suffer from artifactual symmetry breaking.