EXTRA HIGH ACCURACY FITTING OF THE PES FOR SUB-PERCENT CALCULATION OF INTENSITIES

Calculation of rotation-vibration line intensities with sub-percent accuracy has recently become a standard requirement for the applications in retrieval and monitoring of gases in the Earth's atmosphere and potentially in the atmospheres of exoplanets. A major factor in the accurate calculation of intensities is the requirement for a high accuracy \textit{ab initio} Dipole moment surface (DMS) (e.g. references \footnote{L.Lodi, J. Tennyson and O.L. Polyansky, \textit{Journal of Chemical Physics}, \textbf{135}, 034113, (2011) } and \footnote{ O.L. Polyansky, K. Bielska, M. Ghysels, L. Lodi, N.F.Zobov, J.T.Hodges, J. Tennyson \textit{ Physical Review Letters}, \textbf{114}, 243001, (2015) }). We demonstrate here that the change from the ”good” potential energy surface (PES) to ”excellent” PES, used for the intensity calculations is also important. By ”good” we mean here, for example, the PES a standard deviation of 0.025 \wn and by ”excellent” - the PES with the standard deviation 0.011 \wn. Details of studies on \chem{H_2O}\footnote{I.I Mizus, A.A. Kyuberis, N.F. Zobov, V.Y. Makhnev, O.L. Polyansky and J. Tennyson \textit{Phil. Trans. R. Soc. A}, \textbf{376}, 20170149, (2018)}, \chem{O_3} \footnote{O.L.Polyansky, N.F. Zobov, I.I Mizus, A.A. Kyuberis. L. Lodi and J. Tennyson \textit{JQSRT},\textbf{ 210}, 127-135 (2018)}, \chem{HCN} and \chem{CO_2} molecules will be presented in the talk

QED correction for H3+_3^+

A quantum electrodynamics (QED) correction surface for the simplest polyatomic and polyelectronic system H$_3^+$ is computed using an approximate procedure. This surface is used to calculate the shifts to vibration-rotation energy levels due to QED; such shifts have a magnitude of up to 0.25 cm$^{-1}$ for vibrational levels up to 15~000 cm$^{-1}$ and are expected to have an accuracy of about 0.02 cm$^{-1}$. Combining the new H$_3^+$ QED correction surface with existing highly accurate Born-Oppenheimer (BO), relativistic and adiabatic components suggests that deviations of the resulting {\it ab initio} energy levels from observed ones are largely due to non-adiabatic effects

High accuracy calculations of the rotation-vibration spectrum of H3+_3^+

Calculation of the rotation-vibration spectrum of H3+, as well as of its deuterated isotopologues, with near-spectroscopic accuracy requires the development of sophisticated theoretical models, methods, and codes. The present paper reviews the state-of-the-art in these fields. Computation of rovibrational states on a given potential energy surface (PES) has now become standard for triatomic molecules, at least up to intermediate energies, due to developments achieved by the present authors and others. However, highly accurate Born--Oppenheimer energies leading to highly accurate PESs are not accessible even for this two-electron system using conventional electronic structure procedures e.g., configuration-interaction or coupled-cluster techniques with extrapolation to the complete basis set limit). For this purpose highly specialized techniques must be used, e.g., those employing explicitly correlated Gaussians and nonlinear parameter optimizations. It has also become evident that a very dense grid of \ai\ points is required to obtain reliable representations of the computed points extending from the minimum to the asymptotic limits. Furthermore, adiabatic, relativistic, and QED correction terms need to be considered to achieve near-spectroscopic accuracy during calculation of the rotation-vibration spectrum of H3+. The remaining and most intractable problem is then the treatment of the effects of non-adiabatic coupling on the rovibrational energies, which, in the worst cases, may lead to corrections on the order of several \cm. A promising way of handling this difficulty is the further development of effective, motion- or even coordinate-dependent, masses and mass surfaces. Finally, the unresolved challenge of how to describe and elucidate the experimental pre-dissociation spectra of H$_3^+$ and its isotopologues is discussed.Comment: Topical review to be published in J Phys B: At Mol Opt Phy

ExoMol molecular line lists XXX: a complete high-accuracy line list for water

A new line list for H$_2$$^{16}$O is presented. This line list, which is called POKAZATEL, includes transitions between rotation-vibrational energy levels up to 41000 cm$^{-1}$ in energy and is the most complete to date. The potential energy surface (PES) used for producing the line list was obtained by fitting a high-quality ab initio PES to experimental energy levels with energies of 41000 cm$^{-1}$ and for rotational excitations up to $J=5$. The final line list comprises all energy levels up to 41000 cm$^{-1}$ and rotational angular momentum $J$ up to 72. An accurate ab initio dipole moment surface (DMS) was used for the calculation of line intensities and reproduces high-precision experimental intensity data with an accuracy close to 1 %. The final line list uses empirical energy levels whenever they are available, to ensure that line positions are reproduced as accurately as possible. The POKAZATEL line list contains over 5 billion transitions and is available from the ExoMol website (www.exomol.com) and the CDS database

High accuracy CO2_2 line intensities determined from theory and experiment

Atmospheric CO$_2$ concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5\%\ or better. Most available laboratory measurements have uncertainties much larger than this. We report a joint experimental and theoretical study providing rotation-vibration line intensities with the required accuracy. The {\it ab initio} calculations are extendible to all atmospherically important bands of CO$_2$ and to its isotologues. As such they will form the basis for detailed CO$_2$ spectroscopic line lists for future studies.Comment: 5 pages, 2 figures, 1 tabl

Use of the complete basis set limit for computing highly accurate ab initio dipole moments

Calculating dipole moments with high-order basis sets is generally only possible for the light molecules, such as water. A simple, yet highly effective strategy of obtaining high-order dipoles with small, computationally less expensive basis sets is described. Using the finite field method for computing dipoles, energies calculated with small basis sets can be extrapolated to produce dipoles that are comparable to those obtained in high order calculations. The method reduces computational resources by approximately 50% (allowing the calculation of reliable dipole moments for larger molecules) and simultaneously improves the agreement with experimentally measured infrared transition intensities. For atmospherically important molecules which are typically too large to consider the use of large basis sets, this procedure will provide the necessary means of improving calculated spectral intensities by several percent

A room temperature CO2_2 line list with ab initio computed intensities

Atmospheric carbon dioxide concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5% or better. We report a theoretical study providing rotation-vibration line intensities substantially within the required accuracy based on the use of a highly accurate {\it ab initio} dipole moment surface (DMS). The theoretical model developed is used to compute CO$_2$ intensities with uncertainty estimates informed by cross comparing line lists calculated using pairs of potential energy surfaces (PES) and DMS's of similar high quality. This yields lines sensitivities which are utilized in reliability analysis of our results. The final outcome is compared to recent accurate measurements as well as the HITRAN2012 database. Transition frequencies are obtained from effective Hamiltonian calculations to produce a comprehensive line list covering all $^{12}$C$^{16}$O$_2$ transitions below 8000 cm$^{-1}$ and stronger than 10$^{-30}$ cm / molecule at $T=296$~