Fully Atomistic Vibrational Free Energy of Molecular Crystals Using Distributed Multipole Moments

Abstract

Predicting the most stable crystal structure of a given molecule at given temperature and pressure remains challenging. While many crystal structure prediction (CSP) methods focus on identifying the structures with the lowest lattice energy at 0 K, there is growing recognition that accurate predictions require evaluating the vibrational free energy at the temperature(s) of interest. In current workflows, such evaluation is either carried out on a few putative structures using computationally demanding atomistic electronic structure calculations or on a larger number of structures using less expensive and less accurate force field approaches under the rigid-body approximation. There is thus a need for methods that offer a better compromise between accuracy and computational cost. We present, for the first time, lattice dynamics expressions that allow for a fully atomistic treatment of molecular flexibility with a force field commonly used in CSP, where electrostatic interactions are modelled with distributed multipole moments. We implement these expressions in the CrystalDynamics algorithm to calculate dispersion curves, phonon densities of states and vibrational free energies. We demonstrate the application of CrystalDynamics to molecular crystals of three compounds of increasing size and flexibility. The use of analytical derivations results in an efficient evaluation of the free energy of molecular crystals. Our work facilitates the study of relative stability as a function of temperature, enabling the evaluation of enantiotropic transitions between crystal forms. Thanks to its low computational cost, the approach can be integrated into CSP protocols to bridge the gap between static lattice energies at 0 K and temperature-dependent free energies.

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BibTeX RIS

BibTeX

@misc{konstantinopoulos2026fully,
  title = {{Fully Atomistic Vibrational Free Energy of Molecular Crystals Using Distributed Multipole Moments}},
  author = {Stefanos Konstantinopoulos and Claire S. Adjiman},
  howpublished = {ChemRxiv},
  year = {2026},
  month = sep,
  doi = {10.26434/chemrxiv.15006364/v2},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15006364/v2},
  note = {CC BY},
}

RIS

TY  - UNPB
TI  - Fully Atomistic Vibrational Free Energy of Molecular Crystals Using Distributed Multipole Moments
AU  - Stefanos Konstantinopoulos
AU  - Claire S. Adjiman
PY  - 2026
DA  - 2026/09/23
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15006364/v2
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15006364/v2
AB  - Predicting the most stable crystal structure of a given molecule at given temperature and pressure remains challenging. While many crystal structure prediction (CSP) methods focus on identifying the structures with the lowest lattice energy at 0 K, there is growing recognition that accurate predictions require evaluating the vibrational free energy at the temperature(s) of interest. In current workflows, such evaluation is either carried out on a few putative structures using computationally demanding atomistic electronic structure calculations or on a larger number of structures using less expensive and less accurate force field approaches under the rigid-body approximation. There is thus a need for methods that offer a better compromise between accuracy and computational cost. We present, for the first time, lattice dynamics expressions that allow for a fully atomistic treatment of molecular flexibility with a force field commonly used in CSP, where electrostatic interactions are modelled with distributed multipole moments. We implement these expressions in the CrystalDynamics algorithm to calculate dispersion curves, phonon densities of states and vibrational free energies. We demonstrate the application of CrystalDynamics to molecular crystals of three compounds of increasing size and flexibility. The use of analytical derivations results in an efficient evaluation of the free energy of molecular crystals. Our work facilitates the study of relative stability as a function of temperature, enabling the evaluation of enantiotropic transitions between crystal forms. Thanks to its low computational cost, the approach can be integrated into CSP protocols to bridge the gap between static lattice energies at 0 K and temperature-dependent free energies.
ER  -

Abstract

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