Novel methodology for systematically constructing global effective models from ab initio-based surfaces: A new insight into high-resolution molecular spectra analysis

Abstract

In this paper, a novel methodology is presented for the construction of global effective rotation-vibration spectroscopic models from potential energy and transition moment ab initio surfaces. Non-empirical effective Hamiltonians are obtained via the block-diagonalization of selected variationally-computed eigenvector matrices, with the possibility of matching observation without any fit. For the first time, the derivation of effective transition moment operators is also carried out in a systematic way. This general approach can be implemented quite easily in most of the variational computer codes and turns out to be a clear alternative to the rather involved Van Vleck perturbation method. Symmetry is exploited at all stages to translate first-principles calculations into a set of spectroscopic parameters to be further refined on experiment. Using the strategy proposed in this work, we demonstrate on H2CO, PH3, CH4, C2H4 and SF6 how the analysis of infrared spectra can be drastically simplified compared to a more traditional empirical approach and why completeness is no longer an issue.

Unrivaled accuracy in measuring rotational transitions of greenhouse gases: THz CRDS of CF4

“…Here, the numerical method proposed in ref. 19 is preferred to the commonly used Van Vleck transformations because the dipole moment parameters can be obtained from a dipole moment surface (DMS) without performing tedious algebraic calculations (see Section 5.3).…”

“…Here, a similar experimental setup described previously 7 is used, a cavity ring-down spectroscopy (CRDS) configuration has enabled the direct measurement of the absorption coefficient. 8 R (19) to R (30) and Q (32) to Q(34) line clusters are measured both in frequency and intensity with unequalled precision. CRDS THz line frequencies and intensities are included in a global fit of the n 2 polyad (see Section 5.1) allowing an improvement of the RMS deviation and an accurate determination of the m 3,3 dipole moment parameter specific to the n 3 À n 3 CF 4 transitions.…”

From the Automated Calculation of Potential Energy Surfaces to Accurate Infrared Spectra

“…Besides this, very many vibrational states need to be considered for the simulation of a rovibrational spectrum, which leads to polyad schemes or screening approaches. However, in the recent past some progress has been made in the direction of calculating rovibrational spectra of molecules with more than 4 or 5 atoms, which at least allows for a qualitative comparison. , …”

“…However, in the recent past some progress has been made in the direction of calculating rovibrational spectra of molecules with more than 4 or 5 atoms, which at least allows for a qualitative comparison. 76,77 This shall be exemplified by RVCI calculations for formamide, 78 a molecule that can hardly be described by VPT2 calculations (cf. Table 1).…”

New Theoretical Infrared Line List for the Methyl Radical with Accurate Vibrational Band Origins from High-Level Ab Initio Calculations

“…Such approach can provide satisfactory results for the strongest bands (e.g., for ν 2 of CH 3 ) but turns out not sufficient to predict weaker bands whose intensities mainly come from vibrational–rotational interactions. A new, non-perturbative approach was recently proposed in ref to obtain the transition moment spectroscopic effective parameters directly from the ab initio DMS for computing both cold and hot bands.…”