Two Faces of the Two-Phase Thermodynamic Model

• Published in: Journal of chemical theory and computation Vol. 17; no. 11; pp. 7187 - 7194
• Main Authors: Madarász, Ádám, Hamza, Andrea, Ferenc, Dávid, Bakó, Imre
• Format: Journal Article
• Language: English
• Published: Washington American Chemical Society 09-11-2021
• Subjects: Alcohols; Amines; Anharmonicity; Errors; Fluid dynamics; Halocarbons; Harmonic oscillators; Organic liquids; Physical simulation; Self diffusion; Specific heat; Thermodynamic models; Thermodynamics
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/acs.jctc.1c00156

The quantum harmonic model and the two-phase thermodynamic method (2PT) are widely used to obtain quantum-corrected properties such as isobaric heat capacities or molar entropies. 2PT heat capacities were calculated inconsistently in the literature. For water, the classical heat capacity was also considered, but for organic liquids, it was omitted. We reanalyzed the performance of different quantum corrections on the heat capacities of common organic solvents against experimental data. We have pointed out serious flaws in previous 2PT studies. The vibrational density of states was calculated incorrectly causing a 39% relative error in diffusion coefficients and 45% error in the 2PT heat capacities. The wrong conversion of isobaric and isochoric heat capacities also caused about 40% error but in the other direction. We have introduced the concept of anharmonic correction (AC), which is simply the deviation of the classical heat capacity from that of the harmonic oscillator model. This anharmonic contribution is around +30 to 40 J/(mol K) for water depending on the water model and −8 to −10 J/(mol K) for hydrocarbons and halocarbons. AC is unrealistically large, +40 J/(K mol) for alcohols and amines, indicating some deficiency of the OPLS force field. The accuracy of the computations was also assessed with the determination of the self-diffusion coefficients.