The quasi-harmonic free energy is a statistical mechanical quantity that describes the energetics of a system at finite temperature. It takes into account both the potential energy and the thermal energy of a system, and is used to calculate thermodynamic properties such as entropy, heat capacity, and equilibrium constants.
Unlike other free energy models, the quasi-harmonic free energy takes into account the anharmonicity of the system, meaning it considers deviations from harmonic motion. This is important for systems with strong intermolecular interactions or at high temperatures, where anharmonic effects become significant.
The quasi-harmonic free energy is calculated using the partition function, which takes into account the vibrational states of a system. The equation is F = -kT ln(Z), where F is the free energy, k is the Boltzmann constant, T is the temperature, and Z is the partition function.
The quasi-harmonic free energy is used to study the thermodynamics of molecular systems, such as proteins, nucleic acids, and polymers. It is also used in computational chemistry and materials science to predict the structure, stability, and reactivity of molecules and materials at different temperatures.
The quasi-harmonic free energy model assumes that the vibrational modes of a system are independent, which may not always be the case. It also neglects the effects of quantum mechanics, which can be significant for light atoms. Additionally, it is limited to systems in thermal equilibrium and may not accurately describe non-equilibrium processes.