  Calculations of transition intensities for small molecules like H$_2$O, CO,
CO$_2$ based on s high-quality potential energy surface (PES) and dipole moment
surface (DMS) can nowadays reach sub-percent accuracy. An extension of this
treatment to a system with more complicated internal structure -- HCN/HNC
(hydrogen cyanide/hydrogen isocyanide) is presented. A highly accurate
spectroscopically-determined PES is built based on a recent \aipes\ of the
HCN/HNC isomerizing system. 588 levels of HCN with $J$~=~(0,~2,~5,~9,~10) are
reproduced with a standard deviation from the experimental values of
$\sigma=0.0373$ \cm\ and 101 HNC levels with $J$~=~(0,~2) are reproduced with
$\sigma=0.37$ \cm. The dependence of the HCN rovibrational transition
intensities on the PES is tested for the wavenumbers below 7200 \cm.
Intensities are computed using wavefunctions generated from an \ai\ and our
optimized PES. These intensities differ from each other by more than 1\%\ for
about 11\% of the transitions tested, showing the need to use an optimized PES
to obtain wavefunctions for high-accuracy predictions of transition
intensities. An \ai\
  DMS is computed for HCN geometries lying below 11~200 \cm. Intensities for
HCN transitions are calculated using a new fitted PES and newly calculated DMS.
The resulting intensities compare much better with experiment than previous
calculations. In particular, intensities of the H--C stretching and bending
fundamental transitions are predicted with the subpercent accuracy.
