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

BNB is a challenging example of artifactual symmetry-breaking effects due to its susceptibility to a pseudo second-order Jahn–Teller interaction, which results in a structure with unequal BN bondlengths. The fixed-node diffusion Monte Carlo method is employed to calculate the potential curves along the symmetric and asymmetric stretching coordinates. With a multi-determinant wavefunction, the symmetric and asymmetric structures are found degenerate within statistical errors, with the asymmetric configuration lower in energy. The energy difference between the two structures is smaller than the multi-reference coupled cluster result obtained with four determinants, and supports previous conclusions that BNB has a floppy quasi-symmetric ground-state.