Isomerization of Bicyclo[1.1.0]butane by Means of the Diffusion Quantum Monte Carlo Method

The isomerization of bicyclo[1.1.0]butane which comprises species with different multireference character is studied by means of the diffusion quantum Monte Carlo method (DMC). Accurate multireference DMC calculations are presented. It can be shown that at most three configuration state functions are required to achieve a balanced description of dynamical and nondynamical electron correlation. A general scheme is described that promotes efficient error cancellation in multireference DMC calculations.