Ab initio rotation-vibration spectra of HCN and HNC

High Accuracy ab Initio Calculations of Rotational-Vibrational Levels of the HCN/HNC System

Highly accurate ab initio calculations of vibrational and rotational-vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously. An adiabatic Born-Oppenheimer diagonal correction (BODC) correction surface as well as a relativistic correction surface were also calculated. These surfaces were used to compute vibrational and rotational-vibrational energy levels up to 25 000 cm^-1 which reproduce the extensive set of experimentally known HCN/HNC levels with a root-mean-square deviation σ = 1.5 cm^-1. We studied the effect of nonadiabatic effects by introducing opportune radial and angular corrections to the nuclear kinetic energy operator. Empirical determination of two nonadiabatic parameters results in observed energies up to 7000 cm^-1 for four HCN isotopologues (HCN, DCN, H¹³CN, and HC¹⁵N) being reproduced with σ = 0.37 cm^-1. The height of the isomerization barrier, the isomerization energy and the dissociation energy were computed using a number of models; our best results are 16 809.4, 5312.8, and 43 729 cm^-1, respectively.

Photodissociation of HCN and HNC isomers in the 7-10 eV energy range

The ultraviolet photoabsorption spectra of the HCN and HNC isomers have been simulated in the 7-10 eV photon energy range. For this purpose, the three-dimensional adiabatic potential energy surfaces of the 7 lowest electronic states, and the corresponding transition dipole moments, have been calculated, at multireference configuration interaction level. The spectra are calculated with a quantum wave packet method on these adiabatic potential energy surfaces. The spectra for the 3 lower excited states, the dissociative electronic states, correspond essentially to predissociation peaks, most of them through tunneling on the same adiabatic state. The 3 higher electronic states are bound, hereafter electronic bound states, and their spectra consist of delta lines, in the adiabatic approximation. The radiative lifetime towards the ground electronic states of these bound states has been calculated, being longer than 10 ns in all cases, much longer that the characteristic predissociation lifetimes. The spectra of HCN is compared with the available experimental and previous theoretical simulations, while in the case of HNC there are no previous studies to our knowledge. The spectrum for HNC is considerably more intense than that of HCN in the 7-10 eV photon energy range, which points to a higher photodissociation rate for HNC, compared to HCN, in astrophysical environments illuminated by ultraviolet radiation.

Generating accurate dipole moment surfaces using modified Shepard interpolation

We outline an approach for building molecular dipole moment surfaces using modified Shepard interpolation. Our approach is highly automated, requires minimal parameterization, and is iteratively improvable. Using the water molecule as a test case, we investigate how different aspects of the interpolation scheme affect the rate of convergence of calculated IR spectral line intensities. It is found that the interpolation scheme is sensitive to coordinate singularities present at linear geometries. Due to the generally monotonic nature of the dipole moment surface, the one-part weight function is found to be more effective than the more complicated two-part variant, with first-order interpolation also giving better-than-expected results. Almost all sensible schemes for choosing interpolation reference data points are found to exhibit acceptable convergence behavior.

Calculations of rovibrational energies and dipole transition intensities for polyatomic molecules using MULTIMODE

We report rigorous calculations of rovibrational energies and dipole transition intensities for three molecules using a new version of the code MULTIMODE. The key features of this code which permit, for the first time, such calculations for moderately sized but otherwise general polyatomic molecules are briefly described. Calculations for the triatomic molecule BF(2) are done to validate the code. New calculations for H(2)CO and H(2)CS are reported; these make use of semiempirical potentials but ab initio dipole moment surfaces. The new dipole surface for H(2)CO is a full-dimensional fit to the dipole moment obtained with the coupled-cluster with single and double excitations and a perturbative treatment of triple excitations method with the augmented correlation consistent triple zeta basis set. Detailed comparisons are made with experimental results from a fit to relative data for H(2)CS and absolute intensities from the HITRAN database for H(2)CO.

New Double Many-Body Expansion Potential Energy Surface for Ground-State HCN from a Multiproperty Fit to Accurate ab Initio Energies and Rovibrational Calculations

An accurate single-sheeted double many-body expansion potential energy surface has been obtained for the ground electronic state of the hydrogen cyanide molecule via a multiproperty fit to ab initio energies and rovibrational data. This includes 106 rovibrational levels and 2313 discrete points, which are fit with a rmsd of 4 cm(-1) and 2.42 kcal mol(-1), respectively, and seven zero first-derivatives that are reproduced at three stationary points. Since the potential also describes accurately the appropriate asymptotic limits at the various dissociation channels, it is commended both for the simulation of rovibrational spectra and reaction dynamics.