The equilibrium structure and potential energy surface of calcium monohydroxide in its ground doublet state, X 2 Σ + CaOH, have been determined from large-scale ab initio calculations using the spin-restricted coupledcluster method, RCCSD(T), with basis sets of quadruple-and quintuple-quality. The vibrational-rotational energy levels of the CaOH and CaOD isotopomers were calculated using the variational method. The spectroscopic constants determined are found to be in remarkably good agreement with experimental data.

Results are presented for highly accurate ab initio variational calculation of the rotationvibration energy levels of H 2 O 2 in its electronic ground state. These results use a recently computed potential energy surface and the variational nuclear-motion programmes WARV4, which uses an exact kinetic energy (EKE) operator, and TROVE, which uses a numerical expansion for the kinetic energy. The TROVE calculations are performed for levels with high values of rotational excitation, J up to 35. The purely ab initio calculations of the rovibrational energy levels reproduce the observed levels with a standard deviation of about 1 cm −1 , similar to that of the J = 0 calculation as the discrepancy between theory and experiment for rotational energies within a given vibrational state is substantially determined by the error in the vibrational band origin. Minor adjustments are made to the ab initio equilibrium geometry and to the height of the torsional barrier. Using these and correcting the band origins using the error in J = 0 states lowers the standard deviation of the observed − calculated energies to only 0.002 cm −1 for levels up to J = 10 and 0.02 cm −1 for all experimentally know energy levels, which extend up to J = 35.