A new spectroscopically-determined potential energy surface and \emph{ab initio} dipole moment surface for high accuracy HCN intensity calculations

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.