Accurate Prediction of the Ammonia Probes of a Variable
  Proton-to-Electron Mass Ratio
  A comprehensive study of the mass sensitivity of the
vibration-rotation-inversion transitions of $^{14}$NH$_3$, $^{15}$NH$_3$,
$^{14}$ND$_3$, and $^{15}$ND$_3$ is carried out variationally using the TROVE
approach. Variational calculations are robust and accurate, offering a new way
to compute sensitivity coefficients. Particular attention is paid to the
$\Delta k=\pm 3$ transitions between the accidentally coinciding
rotation-inversion energy levels of the $\nu_2=0^+,0^-,1^+$ and $1^-$ states,
and the inversion transitions in the $\nu_4=1$ state affected by the "giant"
$l$-type doubling effect. These transitions exhibit highly anomalous
sensitivities, thus appearing as promising probes of a possible cosmological
variation of the proton-to-electron mass ratio $\mu$. Moreover, a simultaneous
comparison of the calculated sensitivities reveals a sizeable isotopic
dependence which could aid an exclusive ammonia detection.
