PolyMLR: AN ANALYTIC MODEL FOR POLYATOMIC POTENTIALS WITH FEWER UNPHYSICAL PARAMETERS. APPLICATION TO CO2.

Dattani, Nikesh S.

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https://hdl.handle.net/2142/100778 Copy

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Title

PolyMLR: AN ANALYTIC MODEL FOR POLYATOMIC POTENTIALS WITH FEWER UNPHYSICAL PARAMETERS. APPLICATION TO CO2.

Author(s)

Dattani, Nikesh S.

Issue Date

06/20/18

Keyword(s)

Mini-symposium: New Ways of Understanding Molecular Spectra

Abstract

One has to calculate thousands or millions of \textit{ab initio} points for potential energy surfaces even for molecules with only a few atoms. For diatomics, the MLR (Morse/long-range)\footnote{Dattani N. S., Le$\,$Roy R. J., Ross A., Linton C. (2008) Proceedings of the 63rd Annual International Symposium on Molecular Spectroscopy. {\textbf{p301}}}\footnote{Le$\,$Roy R. J., Dattani N. S., Coxon J. A., Ross A. J., Crozet P., Linton C. (2009) \textit{J. Chem Phys.} \textbf{131}, 204309} model has been very successful, making it possible to represent the entire curve accurately with just a few \textit{ab initio} points, or a few spectral lines. With the MLR model it is also possible to extrapolate and interpolate in a way that allows successful predictions of energy level locations several thousand cm$^{-1}$ away from the data region\footnote{Dattani N. S., Le$\,$Roy R. J. (2011) \textit{J. Mol. Spec.}. \textbf{268}, 199-210.; Semczuk M., \textit{et al.} (2013) \textit{Phys. Rev. A} \textbf{87}, 052505}. However no analogous model has existed yet for the intramolecular potentials of polyatomic molecules. A simple model is presented which accurately describes some small molecules with far fewer parameters than previous models, and can be extended to larger molecules too. The benefit of having a good model function is orders of magnitude greater for polyatomics than for diatomics since the amount of data needed for an accurate potential is reduced in each dimension. For example if the calculation of 100 \textit{ab initio} points is reduced to 10 in a diatomic molecule, we may estimate that this factor of 10 reduction in cost becomes at least 10$^{10}$ for a molecule whose potential depends on 10 radial coordinates. As an example, an analytic potential for CO$_2$ is built, which requires fewer parameters than the previous state-of-the-art analytic potential, and obeys the theoretical long-range behavior more closely than all previous potentials, including inclusion of the Axelrod-Teller three-body interaction. The model is based on accurate diatomic potentials representing all atom-atom pairwise interactions, and for CO$_2$, a three-body correction representing the rest of the energy. This emphasizes the value of accurate molecular spectroscopy for simple diatomics, which is sometimes considered to be less interesting than research involving large molecules. Diatomic potentials are valuable as building blocks for large-molecule potentials. An open-source computer program for building PolyMLR potentials for polyatomic molecules is introduced.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

English

Permalink

http://hdl.handle.net/2142/100778

DOI

10.15278/isms.2018.WB04

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