Variational Calculation of Highly Excited Rovibrational Energy Levels of H₂O₂

Abstract

Results are presented for highly accurate ab initio variational calculation of the rotationvibration energy levels of H 2 O 2 in its electronic ground state. These results use a recently computed potential energy surface and the variational nuclear-motion programmes WARV4, which uses an exact kinetic energy (EKE) operator, and TROVE, which uses a numerical expansion for the kinetic energy. The TROVE calculations are performed for levels with high values of rotational excitation, J up to 35. The purely ab initio calculations of the rovibrational energy levels reproduce the observed levels with a standard deviation of about 1 cm −1 , similar to that of the J = 0 calculation as the discrepancy between theory and experiment for rotational energies within a given vibrational state is substantially determined by the error in the vibrational band origin. Minor adjustments are made to the ab initio equilibrium geometry and to the height of the torsional barrier. Using these and correcting the band origins using the error in J = 0 states lowers the standard deviation of the observed − calculated energies to only 0.002 cm −1 for levels up to J = 10 and 0.02 cm −1 for all experimentally know energy levels, which extend up to J = 35.

ExoMol line lists – XV. A new hot line list for hydrogen peroxide

“…Our previous room-temperature H2O2 line list (Al-Refaie et al 2015a) was computed using the ab initio PES of Ma lyszek & Koput (2013) with the small adjustment of the ab initio equilibrium geometry and height of the torsional barrier proposed by Polyansky et al (2013). This PES reproduces the known empirical energy levels with a root mean square (rms) of about 1-2 cm -1 .…”

“…For example the high stretching or bending overtones are poorly represented in the experimental and therefore it is important to retain the ab initio quality of the original PES by Polyansky et al (2013). To this end we constrain the PES around ab initio energies at each geometry (Yurchenko et al 2003(Yurchenko et al , 2011bYachmenev et al 2011;Sousa-Silva et al 2015).…”

“…However, the computational cost is greatly reduced for rotationally excited states by using TROVE. Convergence of the KEO usually requires an expansion order of 6 or 8 (Yurchenko et al 2007); 6 being chosen for this work and 8 for the potential energy expansion as suggested from a previous H2O2 calculation by Polyansky et al (2013).…”

“…The peroxide system was used to benchmark the large amplitude calculations of MULTIMODE (Bowman et al 2003) up to J = 20 and showed good agreement against HITRAN line intensities but the PES used had an rms of ≈ 20 cm -1 against experimental band centers. However, this PES has been superceded by the higher accuracy ab initio potential energy surface (PES) of Ma lyszek & which was further modified by Polyansky et al (2013). This modified PES was used for our room-temperature line list (Al-Refaie et al 2015a) and provides the starting point for the refinements performed here.…”

“…The ExoMol project (Tennyson & Yurchenko 2012) aims to produce comprehensive theoretical molecular line lists to aid in studies of the atmospheres of exoplanets, cool stars and other (hot) bodies. A room temperature line list for H2O2 was previously computed by us (Al-Refaie et al 2015a) using the PES of Polyansky et al (2013) and a new ab initio dipole moment surface (DMS). It provides about 1 billion transitions at up to 8,000 cm -1 .…”