High-accurate intermolecular potential energy surface of HCN−H2 complex with intramolecular vibrational mode of HCN included

Abstract

\begin{wrapfigure}{l}{0pt} \includegraphics[scale=0.35]{oh2-hcn-spec-01.eps} \end{wrapfigure} Hydrogen is one of the most abundant interstellar species. Observation of rotational and vibrational spectra of \chem{H_2} containing complexes is of great importance because they are possible candidates for radio-astronomical detection. \chem{CO}, \chem{HCN}, \chem{HCCH} are as isoelectronic molecules of \chem{N_2}, each with a strong triple bond. It had been a big challenge to predict reliable theoretical rovibrational spectra of complexes including such species because the higher order electron correlation energy plays a non-negligible role in improving the accuracy. However, recent works on \chem{CO-H_2}\footnote{{\it J. Chem. Phys.}, {\bf 139}, 164315 (2013)

{\it Science}, {\bf 336}, 1147 (2012).} have shown that it is possible to reproduce the experimental spectra quantitatively. In this work, we calculate a five-dimension potential energy surface (PES) of \chem{HCN-H_2} complex which explicitly include the intramolecular asymmetric stretching vibrational mode(\chem{C-H},$Q_3$) coordinate at CCSD(T)/aug-cc-pVQZ+$3s3p2d1f1g$ level, corrected with electron correlation energy from the triple and quadruple excitation. Vibrational average over intramolecular vibration mode is made with \chem{HCN} monomer at ground and the first excited vibrational states respectively, and the averaged data are fitted to two four-dimension Morse/Long-Range (MLR) potential energy functions. Based on the MLR PESs, for the first time, we calculated the spectra of \chem{HCN-{\it para}H_2} and \chem{HCN-{\it ortho}H_2}. The results for \chem{HCN-{\it ortho}H_2} are in good agreement with the published experimental data\footnote{{\it J. Chem. Phys.}, {\bf 115}, 5137 (2001).} with root-mean-square-difference (RMSD) only 0.01\wn, which validates the accuracy of the PESs.