Knowledge Bank

Diffusion Monte Carlo (DMC) has been shown to be a highly successful technique for treating quantum zero-point effects of very floppy molecules and clusters. Our group has developed a fixed-node DMC methodology that allows us to expand the application of the approach to studies of rotationally excited states of such systems. We recently applied this approach to the study of H$\mathrm{3+}$.{} We chose this system because of the availability of a global potential energy surface of spectroscopic accuracy, and the results of converged variational calculations have been reported that can be used to assess the accuracy of the DMC calculations. As a symmetric top molecule, the nodal structures of the rotationally excited states of H$\mathrm{3+}$ are well known and can be used in fixed-node DMC calculations. We have recently extended this methodology to asymmetric top molecules, using H$2$D$+$ as a test system for these types of molecules as it has a $\kappa$ value near zero. Here, we describe these extensions and present the results of DMC calculations of representative rotationally excited states of H$2$D$+$.