Rotational memory function of SPC/E water.

Area of Science:

• Condensed matter physics
• Molecular dynamics
• Dielectric spectroscopy

Background:

• Memory effects are crucial for condensed matter dynamics, influencing relaxation processes via memory functions.
• Memory functions for water dipole rotations have not been directly computed from molecular dynamics simulations.

Purpose of the Study:

• To calculate memory functions for single-dipole rotations and collective dipole moment dynamics in SPC/E water.
• To validate theories connecting dielectric spectroscopy, single-particle dynamics, and collective relaxation.

Main Methods:

• Molecular dynamics simulations were employed to compute memory functions.
• Calculations focused on single-dipole rotations and the overall sample dipole moment.

Main Results:

• Normalized memory functions for single-particle and collective dipole dynamics were found to be nearly identical.
• A short memory time (≲1 fs) was determined for water dipole rotations.
• An analytical equation for the rotational memory time was derived.

Conclusions:

• The findings validate theories used in dielectric spectroscopy, particularly the link between collective and single-particle relaxation times.
• The study confirms that the dielectric function does not contain dynamic information beyond that in the single-dipole correlation function.
• Rotational diffusion models are justified for describing single molecular dipole dynamics in bulk water due to the short memory time.