Embedded fragmentation of vibrational energies

Area of Science:

• * Quantum Chemistry
• * Computational Chemistry
• * Molecular Modeling

Background:

• * Evaluating zero-point vibrational energies (ZPVE) for large systems like molecular clusters and crystals is computationally intensive.
• * Traditional methods struggle to accurately capture the contributions of delocalized acoustic phonons.

Purpose of the Study:

• * To investigate if ZPVE of molecular clusters and crystals can be accurately computed by summing ZPVE of embedded molecular fragments.
• * To determine the optimal fragmentation unit: monomers or overlapping dimers.
• * To assess the feasibility of including acoustic phonon contributions.

Main Methods:

• * Application of embedded monomer- and dimer-fragmentation methods.
• * Harmonic ZPVE calculations on hydrogen fluoride clusters, hydrogen fluoride crystal, and water clusters.
• * Analysis of fragment embedding effects on vibrational modes.

Main Results:

• * Both monomer and dimer fragmentation schemes accurately reproduce ZPVE, with errors within a few percent or tenths of a kcal/mol.
• * Monomer and dimer methods show similar accuracy for absolute ZPVE, while dimer fragmentation is more reliable for relative ZPVE of isomers.
• * Embedding fields are crucial for accounting for pseudo-translational and librational motions.
• * Imaginary frequencies in fragment modes indicate potential errors in the real part of ZPVE.

Conclusions:

• * Fragment-based approaches, particularly embedded monomer and dimer methods, are effective for calculating ZPVE in molecular clusters and crystals.
• * The choice between monomer and dimer fragmentation depends on whether absolute or relative ZPVE values are prioritized.
• * The electrostatic environment and embedding effects are critical for accurate ZPVE calculations.
• * The study provides a robust computational strategy for ZPVE determination in condensed-phase systems.