## A code for analysis of the fine structure in near-rigid weakly-bonded open-shell complexes that consist of a diatomic radical in a $$^3 \Sigma$$ state and a closed-shell molecule.(English)Zbl 1219.82011

Summary: A FORTRAN code is developed for simulation and fitting the fine structure of a planar weakly-bonded open-shell complex that consists of a diatomic radical in a $$^3\Sigma$$ electronic state and a diatomic or a polyatomic closed-shell molecule. The program sets up the proper total Hamiltonian matrix for a given $$J$$ value and takes account of electron-spin-electron-spin, electron-spin rotation interactions, and the quartic and sextic centrifugal distortion terms within the complex. Also, $$R$$-dependence of electron-spin-electron-spin and electron-spin rotation couplings are considered. The code does not take account of effects of large-amplitude internal rotation of the diatomic radical within the complex. It is assumed that the complex has a well defined equilibrium geometry so that effects of large amplitude motion are negligible. Therefore, the computer code is suitable for a near-rigid rotor. Numerical diagonalization of the matrix provides the eigenvalues and the eigenfunctions that are necessary for calculating energy levels, frequencies, relative intensities of infrared or microwave transitions, and expectation values of the quantum numbers within the complex. Goodness of all the quantum numbers, with exception of $$J$$ and parity, depends on relative sizes of the product of the rotational constants and quantum numbers (i.e., BJ, CJ, and AK), electron-spin-electron-spin, and electron-spin rotation couplings, as well as the geometry of the complex. Therefore, expectation values of the quantum numbers are calculated in the eigenfunctions basis of the complex. The computational time for the least squares fits has been significantly reduced by using the Hellman-Feynman theory for calculating the derivatives. The computer code is useful for analysis of high resolution infrared and microwave spectra of a planar near-rigid weakly-bonded open-shell complex that contains a diatomic fragment in a $$^3\Sigma$$ electronic state and a closed-shell molecule. The computer program was successfully applied to analysis and fitting the observed high resolution infrared spectra of the O$$_2$$HF/O$$_{2}$$DF and O$$_{2}$$N$$_{2}$$O complexes. Test input file for simulation and fitting the high resolution infrared spectrum of the O$$_{2}$$DF complex is provided.

### MSC:

 82-04 Software, source code, etc. for problems pertaining to statistical mechanics 82-08 Computational methods (statistical mechanics) (MSC2010) 81V55 Molecular physics 81Q10 Selfadjoint operator theory in quantum theory, including spectral analysis

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### References:

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