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

Abstract Calculations of transition intensities for small molecules like H2O, CO, CO2 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 ab initio PES 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 σ = 0.0373 cm−1 and 101 HNC levels with J = (0, 2) are reproduced with σ = 0.37 cm−1. The dependence of the HCN rovibrational transition intensities on the PES is tested for the wavenumbers below 7200 cm−1. Intensities are computed using wavefunctions generated from an ab initio 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 ab initio DMS is computed for HCN geometries lying below 11,200 cm−1. 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.