Insights from the Absorption Coefficient for the Development of Polarizable (Multipole) Force Fields

Area of Science:

• Computational chemistry
• Spectroscopy
• Condensed matter physics

Background:

• Terahertz (THz) spectroscopy is crucial for understanding water's dynamic properties.
• Accurate modeling of water's THz absorption requires sophisticated potential models.
• The influence of dipole moments (permanent and induced) on THz spectra is not fully understood.

Purpose of the Study:

• To investigate THz absorption coefficients across various water models.
• To elucidate the role of permanent and induced dipole moments in THz spectra.
• To analyze the contributions of inter- and intramolecular interactions to THz absorption peaks.

Main Methods:

• Simulations of water models: SPC/E (non-polarizable), SWM4-NDP, OPC3-pol, IPOL-0.13, and AMOEBA14 (polarizable).
• Calculation of absorption coefficients in the THz region.
• Spectral decomposition to identify contributions from different interactions.

Main Results:

• Induced dipoles significantly contribute to the 200 cm^-1 spectral peak.
• Enhanced induced dipoles alone do not guarantee experimental agreement.
• Intermolecular interactions dominate the low-frequency THz absorption.
• Intramolecular dipole-dipole interactions explain the 600 cm^-1 peak.

Conclusions:

• Water model choice critically impacts THz spectral predictions.
• A balance between permanent and induced dipoles, alongside accurate intermolecular potentials, is necessary for reproducing experimental THz spectra.
• Dissecting spectral contributions aids in understanding the complex dynamics of water.