Molecular Dynamics (MD) Simulations

Computer simulation has proven to be an indispensable tool for investigating the behavior of liquids. Simulations have successfully reproduced many of the macroscopic and microscopic properties of liquids. Analytical techniques are limited in that they typically do not give a detailed molecular description of the system being investigated.

In MD simulations, the positions, orientations, velocities, and forces on a system of molecules can be computed at each time step (typically about 1 fs). The system evolves according to Newton's laws of motion. The connection to the dynamics of the system is made via time correlation functions (TCFs) of various quantities in the system. A particularly important property is the evolution of the total dipole moment of the system, since it is related to the experimentally observable absorption in the infrared region of the spectrum. By computing this and other dynamical properties, we can compare our simulated spectra to that observed experimentally and can thereby assess the accuracy of the simulations. Additional dynamical quantities that are not experimentally observable, such as the single dipole TCF of a molecule, provide additional insight into the nature of the mixtures.

The configuration of molecules in the system is known at each time step. Therefore, the structure of the molecules in the liquid can be analyzed. We have been especially interested in determining how hydrogen bonding changes in a liquid upon mixing. Having information about both the structure and the dynamics from the same source allows us to connect changes in the structure to changes in the dynamics of the liquid. The figure shows typical methanol chains conformations in methanol/acetone mixtures. We find that they typically have very few branches and adopt prolate shapes.