  Results are presented for highly accurate ab initio variational calculation
of the rotation - vibration energy levels of H2O2 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 \ai\ calculations of the
rovibrational energy levels reproduce the observed levels with a standard
deviation of about 1 \cm, 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\
for levels up to $J = 10$ and 0.02 \cm\ for all experimentally know energy
levels, which extend up to $J = 35$.
