  Modeling of molecular emission from interstellar clouds requires the
calculation of rates for excitation by collisions with the most abundant
species. The present paper focuses on the calculation of rate coefficients for
rotational excitation of the HCN and HNC molecules in their ground vibrational
state in collision with He. The calculations are based on new two-dimensional
potential energy surfaces obtained from highly correlated \textit{ab initio}
calculations. Calculations of pure rotational (de)excitation cross sections of
HCN and HNC by He were performed using the essentially exact close-coupling
method. Cross sections for transitions among the 8 first rotational levels of
HCN and HNC were calculated for kinetic energies up to 1000 cm$^{-1}$. These
cross sections were used to determine collisional rate constants for
temperatures ranging from 5 K to 100 K. A propensity for even $\Delta j$
transitions is observed in the case of HCN--He collisions whereas a propensity
for odd $\Delta j$ transitions is observed in the case of HNC--He collisions.
The consequences for astrophysical models are evaluated and it is shown that
the use of HCN rate coefficients to interpret HNC observations can lead to
significant inaccuracies in the determination of the HNC abundance, in
particular in cold dark clouds for which the new HNC rates show that the
$j=1-0$ line of this species will be more easily excited by collisions than
HCN. An important result of the new HNC-He rates is that the HNC/HCN abundance
ratio derived from observations in cold clouds has to be revised from $>$1 to
$\simeq$1, in good agreement with detailed chemical models available in the
literature.
