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Toward Reliable and Insightful Entropy Calculations on Flexible Molecules.

Natalia Díaz1, Dimas Suárez1

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Accurate molecular entropy calculations are achieved by combining rigid-rotor-harmonic-oscillator (RRHO) entropy with conformational entropy. This method, using extensive molecular dynamics simulations, effectively captures torsional correlations for reliable predictions.

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Area of Science:

  • Computational Chemistry
  • Molecular Modeling
  • Thermodynamics

Background:

  • Absolute molecular entropy is crucial for chemical thermodynamics.
  • Approximating entropy involves rigid-rotor-harmonic-oscillator (RRHO) and conformational contributions.
  • Accurately modeling conformational entropy, especially torsional correlations, remains a challenge.

Purpose of the Study:

  • To develop and validate a robust method for calculating the absolute entropy of flexible molecules.
  • To assess the accuracy of combining RRHO and Gibbs-Shannon conformational entropy.
  • To investigate the impact of torsional motion correlations on entropy calculations.

Main Methods:

  • Partitioning absolute entropy into RRHO and conformational components.
  • Employing discretization and expansion techniques for conformational entropy estimation.
  • Utilizing microsecond-timescale molecular dynamics simulations for extensive sampling.
  • Applying the correlation-consistent multibody local approximation (CC-MLA) for complex systems.

Main Results:

  • Accurate gas-phase entropy predictions for organic molecules with a mean unsigned error of 0.9 cal/(mol K).
  • High accuracy achieved for flexible linear alkanes (C14H30, C16H34, C18H38) with errors below 1 cal/(mol K).
  • Well-converged entropies obtained for drug molecules and ubiquitin, demonstrating the method's applicability to complex systems with significant correlation effects.

Conclusions:

  • The combined RRHO and conformational entropy approach provides accurate molecular entropy calculations.
  • Extensive molecular dynamics simulations and advanced expansion techniques are key to capturing torsional correlations.
  • The CC-MLA approach offers valuable insights into coupled torsional motions and is applicable to large biomolecules.