Fast electron-correlation methods for molecular crystals: an application to the alpha, beta(1), and beta(2)...

Area of Science:

• Solid-state chemistry
• Computational materials science
• Quantum chemistry

Background:

• Accurate prediction of molecular crystal properties is crucial for materials design.
• High-accuracy electron-correlation theories are computationally demanding for periodic systems.
• Understanding polymorphism and spectral assignments requires reliable theoretical models.

Purpose of the Study:

• To develop a routine first-principles method for determining energies, structures, and phonons of molecular crystals.
• To apply the method to solid formic acid and resolve ambiguities in its polymorphic forms.
• To provide accurate assignments for infrared, Raman, and inelastic neutron scattering spectral bands.

Main Methods:

• Approximation of crystal energy using monomer and dimer energies in a polarizable embedding field.
• Efficient computation of first and second energy derivatives with long-range electrostatic correction.
• Application of second-order perturbation theory or higher to model polymorphic structures.

Main Results:

• Accurate energies, structural parameters, and frequencies obtained for beta(1), beta(2), and alpha forms of solid formic acid.
• Reliable assignments proposed for spectral bands based on computed vibrational frequencies.
• Observed spectral data are consistent with the beta(1) form; beta(2) and alpha forms are predicted to be incompatible with low-frequency spectra.

Conclusions:

• The developed method enables routine, high-accuracy determination of molecular crystal properties.
• Infrared band splitting in solid formic acid is attributed to molecular vibrations, not polymorphism.
• The study provides a robust framework for analyzing crystal structures and spectra, aiding in materials characterization.