Disentangling the Entropy of Liquids of Flexible Molecules

Entropy and energy are fundamental quantities that determine the stability of a molecular system. While energy may be calculated from force fields or electronic structure methods, there is still no general method to calculate... [ view full abstract ]

Entropy and energy are fundamental quantities that determine the stability of a molecular system. While energy may be calculated from force fields or electronic structure methods, there is still no general method to calculate entropy. Previously we had proposed methods to calculate the entropy of a single flexible molecule [1] or the entropy of a liquid of rigid molecules [2]. Here we combine these methods to present a new way to calculate the entropy for a liquid of flexible molecules. The force-torque covariance matrix is constructed from the forces measured in a molecular dynamics simulation for all molecules of a given type. The eigenvalues of this matrix are converted to vibrational frequencies and entropy in the harmonic approximation. The entropy arising from the number of rotational energy wells is derived from the molecular coordination number. Computed entropies for a range of organic liquids are found to be in reasonable agreement with experiment and entropies of vaporisation computed from free-energy differences.

1. U. Hensen, F. Grater, R. H. Henchman., J. Chem. Theory Comput., 2014, 10, 4777.

2. R. H. Henchman, J. Chem. Phys., 2007, 126, 064504.