Accurate vibrational properties are crucial for the modeling of molecular crystals, especially for low-frequency modes describing intermolecular motions. Such modes significantly contribute to the Gibbs free energy, which makes them vital for the description of relative polymorph stabilities. Furthermore, calculated low-frequency phonon modes can help to interpret or assign peaks in experimental THz spectra, which are used to distinguish between polymorphs or to detect drugs and explosives. However, routine calculations of practically relevant molecular crystals are essentially limited to density functional theory (DFT) calculations employing the harmonic or quasi-harmonic approximation. Since at room temperature anharmonic effects become increasingly important, an affordable but still accurate method for the calculation of anharmonic effects for periodic systems is desirable.<br/>Herein, we present a new methodology for the calculation of such anharmonic vibrational/phonon frequencies based on a subtractive embedding scheme utilizing molecular fragments (e.g., monomers, dimers, etc.). In this approach, the full periodic system is calculated within the harmonic approximation, and contributions of monomers and dimers within a certain cutoff distance are calculated on an anharmonic level. This is achieved by employing a combination of second-order vibrational perturbation theory (VPT2) and hindered rotor models. Here, anharmonic calculations are only necessary for molecular fragments instead of the full periodic system.<br/>First, we present and discuss benchmark results for this anharmonic approach calculated with CCSD(T) based on a diverse set of small molecular dimers with representative intermolecular interactions. Next, we evaluate the accuracy of various density functionals compared to the obtained CCSD(T) results. Finally, we showcase the accuracy of our embedding approach for several molecular crystals by comparing DFT calculations with experimental values. This methodology can be applied to practically relevant molecular crystals and is expected to improve the description of THz spectra and polymorphic energy landscapes.

Anharmonic vibrational properties of molecular crystals via embedding methods

ACS Spring 2022

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