  Two new nine-dimensional potential energy surfaces (PESs) have been generated
using high-level \textit{ab initio} theory for the two main isotopologues of
methyl chloride, CH$_{3}{}^{35}$Cl and CH$_{3}{}^{37}$Cl. The respective PESs,
CBS-35$^{\,\mathrm{HL}}$ and CBS-37$^{\,\mathrm{HL}}$, are based on explicitly
correlated coupled cluster calculations with extrapolation to the complete
basis set (CBS) limit, and incorporate a range of higher-level (HL) additive
energy corrections to account for core-valence electron correlation,
higher-order coupled cluster terms, scalar relativistic effects, and diagonal
Born-Oppenheimer corrections. Variational calculations of the vibrational
energy levels were performed using the computer program TROVE, whose
functionality has been extended to handle molecules of the form XY$_3$Z. Fully
converged energies were obtained by means of a complete vibrational basis set
extrapolation. The CBS-35$^{\,\mathrm{HL}}$ and CBS-37$^{\,\mathrm{HL}}$ PESs
reproduce the fundamental term values with root-mean-square errors of $0.75$
and $1.00{\,}$cm$^{-1}$ respectively. An analysis of the combined effect of the
HL corrections and CBS extrapolation on the vibrational wavenumbers indicates
that both are needed to compute accurate theoretical results for methyl
chloride. We believe that it would be extremely challenging to go beyond the
accuracy currently achieved for CH$_3$Cl without empirical refinement of the
respective PESs.
