Free energy calculations for a flexible water model

Area of Science:

• Computational chemistry
• Thermodynamics
• Materials science

Background:

• Calculating classical free energies for molecular models with intramolecular flexibility is computationally challenging.
• Accurate phase diagrams require precise accounting for both intermolecular and intramolecular motions.

Purpose of the Study:

• To develop and apply robust computational methods for determining the Gibbs free energy of solids and liquids for flexible molecular models.
• To investigate the low-pressure classical phase diagram of the flexible q-TIP4P/F water model.
• To analyze the influence of intramolecular flexibility on the computed phase diagram.

Main Methods:

• Thermodynamic integration from a fully-interacting solid to a Debye crystal reference state for solid free energy calculation.
• Thermodynamic integration to a Lennard-Jones reference fluid coupled with direct evaluation of the ro-vibrational partition function for liquid free energy calculation.
• Comparison with an underlying rigid-body water model to assess the impact of flexibility.

Main Results:

• A straightforward route to calculating the Gibbs free energy of solids using anisotropic harmonic interactions.
• The flexible q-TIP4P/F water model reasonably describes the ice-I/liquid and ice-III/liquid coexistence lines.
• The ice-II phase is predicted to be metastable for the flexible q-TIP4P/F water model.

Conclusions:

• The developed thermodynamic integration methods provide accurate free energy calculations for flexible molecular systems.
• Intramolecular flexibility significantly influences the computed phase diagram of water models.
• The flexible q-TIP4P/F model offers a reasonable, though not perfect, representation of water's low-pressure solid phases and liquid coexistence.