A Multimer Embedding Approach for Molecular Crystals up to Harmonic Vibrational Properties

TL;DR

This paper introduces a multimer embedding approach that extends existing methods to include trimer interactions, enabling accurate calculations of lattice energies, cell volumes, and vibrational properties of molecular crystals with reduced computational cost.

Contribution

The authors develop a multimer embedding method that incorporates trimer interactions and harmonic vibrational properties, improving accuracy over previous dimer-based approaches.

Findings

Trimer interactions are essential for accurate lattice energy calculations within 1 kJ/mol.

Vibrational properties are well captured at monomer and dimer levels, enabling precise vibrational free energy estimates.

The approach reduces computational costs while maintaining high accuracy for molecular crystal properties.

Abstract

Accurate calculations of molecular crystals are crucial for drug design and crystal engineering. However, periodic high-level density functional calculations using hybrid functionals are often prohibitively expensive for relevant systems. These expensive periodic calculations can be circumvented by the usage of embedding methods in which for instance the periodic calculation is only performed at a lower-cost level and then monomer energies and dimer interactions are replaced by those of the higher-level method. Herein, we extend upon such a multimer embedding approach to enable energy corrections for trimer interactions and the calculation of harmonic vibrational properties up to the dimer level. We evaluate this approach for the X23 benchmark set of molecular crystals by approximating a periodic hybrid density functional (PBE0+MBD) by embedding multimers into less expensive calculations using a generalized-gradient approximation (GGA) functional (PBE+MBD). We show that trimer interactions are crucial for accurately approximating lattice energies within 1 kJ/mol and might also be needed for further improvement of lattice constants and hence cell volumes. Finally, vibrational properties are already very well captured at the monomer and dimer level, making it possible to approximate vibrational free energies at room temperature within 1 kJ/mol.