What It Takes to Compute Highly Accurate Rovibrational Line Lists for Use in Astrochemistry

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: We review the Best Theory + Reliable High-Resolution Experiment (BTRHE) strategy for obtaining highly accurate molecular rovibrational line lists with InfraRed (IR) intensities. The need for highly accurate molecular rovibrational line lists is twofold: (a) assignment of the many rovibrational lines for common stable molecules especially those that exhibit a large amplitude motion, such as NH 3 , or have a high density of states such as SO 2 ; and (b) characterization of the atmospheres of exoplanets, which will be one of the main areas of research in astronomy in the coming decades. The first motivation arises due to the need to eliminate lines due to common molecules in an astronomical observation in order to identify lines from new molecules, while the second motivation arises due to the need to obtain accurate molecular opacities in order to characterize the atmosphere of an exoplanet. The BTRHE strategy first consists of using high-quality ab initio quantum-chemical methods to obtain a global potential energy surface (PES) and dipole moment surface (DMS) that contains the proper physics. The global PES is then refined using a subset of the reliable high-resolution experimental data. The refined PES then gives energy-level predictions to an accuracy similar to the reproduction accuracy of the experimental data used in the refinement step in the interpolation region (i.e., within the range of the experimental data used in the refinement step). The accuracy of the energy levels will slowly degrade as they are extrapolated to spectral regions beyond the high-resolution experimental data used in the refinement step. However, because the degradation is slow, the predicted energy levels can be used to assign new high-resolution experiments, and the data from these can then be used in a subsequent refinement step. In this way, the global PES eventually can yield highly accurate energy levels for all desired spectral regions including to very high energies and high J values. We show that IR intensities computed with the BTRHE rovibrational wave functions and the DMS can be very accurate provided one has minimized the fitting error of the DMS and tested the completeness of the DMS. Some examples of our work on NH 3 , CO 2 , and SO 2 are given to highlight the usefulness of the BTRHE strategy and to provide ideas on how to further improve its predictive power in the future. In particular, it is shown how successive refinement steps, once new high-resolution data are available, can lead to PESs that yield highly accurate transition energies to larger spectral regions. The importance of including nonadiabatic corrections to reduce the J-dependence of errors for H-containing molecules is shown with work on NH 3 . Another very important aspect of the BTRHE approach is the consistency across isotopologues, which allows for highly accurate line lists for any isotopologue once one is obtained for the main isotopologue (which has more high-resolution data available for refineme...

Emerging quantum computing algorithms for quantum chemistry

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“…Improving over the harmonic approximation requires retaining higher-order or anharmonic terms in V , which poses additional challenges, especially a proper choice of curvilinear nuclear coordinates, and developing accurate solvers for the nuclear Schrödinger equation. [100][101][102][103][104][105] Computing solutions of the nuclear Schrödinger equation has many valuable applications, notably supporting the study of chemical processes occurring in atmospheric chemistry, and the identification of molecules in the interstellar medium and circumstellar envelopes. 100,101,[106][107][108][109][110][111]…”

“…[100][101][102][103][104][105] Computing solutions of the nuclear Schrödinger equation has many valuable applications, notably supporting the study of chemical processes occurring in atmospheric chemistry, and the identification of molecules in the interstellar medium and circumstellar envelopes. 100,101,[106][107][108][109][110][111]…”

“…Within the Born–Oppenheimer approximation, the nuclear Hamiltonian has the form where denotes the ground‐state potential energy surface of the electronic Hamiltonian Equation () when nuclei have positions , and are the nuclear masses, in atomic units. Solving the nuclear Schrödinger equation presents additional challenges: first of all, as the interaction among nuclei is mediated by electrons, the function is not known a priori, and needs to be computed from quantum chemical calculations at fixed nuclear geometries as in Section 3.1, and then fitted to an appropriate functional form, which can be an expensive procedure 100,101 . For certain problems, the harmonic oscillator approximation of is appropriate, which is given by 100 In Equation () …”

“…As a result of equal spacing of energy levels for a given normal mode, all transitions occur at the same frequency, and bond dissociation is not described. Improving over the harmonic approximation requires retaining higher‐order or anharmonic terms in , which poses additional challenges, especially a proper choice of curvilinear nuclear coordinates, and developing accurate solvers for the nuclear Schrödinger equation 100–105 …”