Large-Scale Calculation of Vibrational Sum Frequency Generation Spectra of Aqueous Interfaces

Area of Science:

• Interface science
• Computational chemistry
• Spectroscopy

Background:

• Vibrational sum-frequency generation (vSFG) spectroscopy is crucial for studying interfacial molecular structures.
• Calculating the phase-resolved resonant vSFG susceptibility (χ(2)(ω)) is computationally intensive and requires specialized tools.
• Understanding water and hydroxyl behavior at various interfaces is key in fields ranging from materials science to biology.

Purpose of the Study:

• To present a novel software package for calculating the phase-resolved resonant vSFG susceptibility (χ(2)(ω)) of water and hydroxyl groups at planar interfaces.
• To provide a versatile tool compatible with various molecular dynamics (MD) simulation outputs (AIMD and CMD).
• To enable flexible interface modeling and analysis of diverse aqueous systems.

Main Methods:

• Developed a FORTRAN-based software, parallelized with OpenMP, optimized for memory efficiency.
• Implemented functionality to read multiple molecular trajectory formats (AIMD, CMD).
• Included features for calculating instantaneous surfaces, handling large systems (tens of thousands of atoms), and supporting unlimited simulation times.

Main Results:

• The software successfully calculates vSFG susceptibility for water and hydroxyls at air/water, quartz/aqueous solution, and alumina/aqueous solution interfaces.
• Demonstrated the impact of various MD simulation parameters (e.g., simulation length, system size) and code parameters (e.g., filter relaxation) on the results.
• Provided representative results for different interface types, validating the software's capabilities.

Conclusions:

• The released software offers a robust and flexible platform for calculating interfacial vSFG susceptibility.
• It facilitates detailed analysis of molecular dynamics simulations for a wide range of aqueous interfaces.
• Future extensions aim to differentiate signals from specific molecular bonds, further enhancing its utility.