using fixednode diffusion monte carlo to investigate the effects of rotationvibration coupling in highly fluxional asymmetric top molecules application to h2d a fixednode diffusion monte carlo approach for obtaining the energies and wave functions of the rotationally excited states of asymmetric top molecules that undergo large amplitude zeropoint vibrational motions is reported the nodal surfaces required to introduce rotational excitation into the diffusion monte carlo calculations are obtained from the roots of the asymmetric top rigid rotor wave functions calculated using the systems zeropoint vibrationally averaged rotational constants using h2d as a model system the overall accuracy of the methodology is tested by comparing to the results of converged variational calculations the ability of the fixednode diffusion monte carlo approach to provide insights into the nature and strength of the rotationvibration coupling present in the rotationally excited states of highly fluxional asymmetric tops is discussed finally the sensitivity of the methodology to the details of its implementation such as the choice of embedding scheme is explored