Extension of the MIRS computer package for the modeling of molecular spectra : from effective to full ab initio ro-vibrational hamiltonians in irreducible tensor form

TL;DR

The paper describes an enhanced version of the MIRS software that now supports full ab initio ro-vibrational Hamiltonians and improved computational capabilities for modeling molecular spectra.

Contribution

It introduces advanced algorithms and features in MIRS for handling larger polyads, overlapping polyads, and direct ab initio predictions, surpassing previous limitations.

Findings

Successfully applied to spectra of various molecules like CH₃D, CH₄, CH₃Cl, CH₃F, and PH₃.

Enhanced computational efficiency and convergence in spectral data fitting.

Extended applicability to full ab initio Hamiltonians beyond effective polyad models.

Abstract

The MIRS software for the modeling of ro-vibrational spectra of polyatomic molecules was considerably extended and improved. The original version (Nikitin, et al. JQSRT, 2003, pp. 239--249) was especially designed for separate or simultaneous treatments of complex band systems of polyatomic molecules. It was set up in the frame of effective polyad models by using algorithms based on advanced group theory algebra to take full account of symmetry properties. It has been successfully used for predictions and data fitting (positions and intensities) of numerous spectra of symmetric and spherical top molecules within the vibration extrapolation scheme. The new version offers more advanced possibilities for spectra calculations and modeling by getting rid of several previous limitations particularly for the size of polyads and the number of tensors involved. It allows dealing with overlapping polyads and includes more efficient and faster algorithms for the calculation of coefficients related to molecular symmetry properties (6C, 9C and 12C symbols for C_{3v}, T_{d}, and O_{h} point groups) and for better convergence of least-square-fit iterations as well. The new version is not limited to polyad effective models. It also allows direct predictions using full ab initio ro-vibrational normal mode hamiltonians converted into the irreducible tensor form. Illustrative examples on CH_{3} D, CH_{4}, CH_{3} Cl, CH_{3} F and PH_{3} are reported reflecting the present status of data available. It is written in C++ for standard PC computer operating under Windows. The full package including on-line documentation and recent data are freely available at [http://www.iao.ru/mirs/mirs.htm] or [http://xeon.univ-reims.fr/Mirs/||http://xeon.univ-reims.fr/Mirs/] or [http://icb.u-bourgogne.fr/OMR/SMA/SHTDS/MIRS.html].