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Anharmonic effects in the quantum cluster equilibrium method
Michael von Domaros1, Eva Perlt1
1Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstr. 6, D-53115 Bonn, Germany.
The Journal of Chemical Physics
|April 8, 2017
Summary
This study introduces anharmonicity into the quantum cluster equilibrium (QCE) model for liquids. Accounting for these vibrational effects improves the calculation of thermodynamic properties for liquid hydrogen chloride.
Area of Science:
- Computational Chemistry
- Physical Chemistry
- Statistical Thermodynamics
Background:
- The quantum cluster equilibrium (QCE) model uses ab initio calculations for finite clusters to model macroscopic liquid phases.
- Previous QCE applications relied on the harmonic approximation for calculating partition functions.
- Anharmonicity, a deviation from harmonic vibrations, is significant in molecular systems and can impact thermodynamic properties.
Purpose of the Study:
- To incorporate anharmonic vibrational effects into the quantum cluster equilibrium (QCE) model.
- To evaluate the impact of anharmonicity on the accuracy of calculated thermodynamic properties for liquid phases.
- To implement an analytical approximation for the Morse partition function within the QCE framework.
Main Methods:
- Implemented an analytical approximation for the Morse partition function to account for anharmonicity.
- Calculated temperature derivatives of the logarithm of the Morse partition function.
- Applied the anharmonic QCE approach to liquid hydrogen chloride (HCl) and its cluster distributions.
Main Results:
- Calculated molar volume, volumetric thermal expansion coefficient, and isobaric heat capacity for liquid HCl.
- Observed improved agreement between calculated and experimental properties when anharmonic effects were included.
- Demonstrated the significance of anharmonicity for accurate thermodynamic predictions in liquid systems.
Conclusions:
- The anharmonic quantum cluster equilibrium (QCE) approach provides a more accurate description of liquid properties compared to the harmonic approximation.
- Accounting for vibrational anharmonicity is crucial for precise thermodynamic calculations of condensed phases.
- The implemented analytical Morse partition function approximation is effective for incorporating anharmonicity into QCE models.
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