Ab-Initio and Molecular Dynamics Methods

Molecular Dynamics (MD) is an atomistic simulation method for studying nanoscale phenomenon in wide range of materials like metals, ceramics and biological molecules. The method typically describes the forces between atoms, using interatomic potentials and tracks the trajectories of as they move due to the attractive and repulsive forces between neighboring atoms using Newton’s Second Law of Motion. When an ensemble of atoms is subject to the desired temperature and pressure, the trajectories are computed by numerical integration of the equations of motion. Based on these calculated trajectories, transport properties, structural characteristics and reaction pathways can be predicted for various materials, liquids and gas mixtures. Using modern computing processors and parallel processing techniques, MD simulations can calculate systems with millions of atoms, large enough to model complex material systems and chemically reactions, for time periods up to 10′s of nanoseconds, long enough to calculate time averaged transport properties and kinetics.

Figure 1: (Left) ACT’s MD simulation of a melting process is shown. Heat applied to the left side raises the temperature of the atoms to above their melting point, causing them to diffuse as a liquid. Molecules on the right side remained below the freezing point, causing them to remain in a lattice structure while oscillating. (Right) ACT’s MD simulation of a carbon nanotube pull-out experiment from an embedded phenolic resin matrix.

(Left) ACT’s MD simulation of a melting process is shown. Heat applied to the left side raises the temperature of the atoms to above their melting point, causing them to diffuse as a liquid. Molecules on the right side remained below the freezing point, causing them to remain in a lattice structure while oscillating.
(Right) ACT’s MD simulation of a carbon nanotube pull-out experiment from an embedded phenolic resin matrix.

Ab initio is Latin for “from the beginning”. Ab initio methods are a class of quantum mechanics based methods which investigate structural, quantum and chemical behavior of the material by solving for Schrödinger equation using different approximations.  Compared to MD, ab initio does not make significant approximations to the electrostatic forces between atoms. As such, ab initio simulations are typically limited to systems with 10’s and 100’s of atoms. The ab-initio simulations involve electronic structure calculations to find the ground energy state of the material system (in absence of temperature input). Still, ab initio simulations can reveal useful information such as interfacial energies and restructuring characteristics. More recently, a new technique called ab initio molecular dynamics (AB MD) has been developed where the accuracy of the interatomic force calculation can be maintained while allowing the atoms to move. This new technique essentially eliminates the force potential approximations in traditional MD. Although a powerful simulation technique, AB MD simulations are limited in the system size and time scale.

ACT’s research team frequently uses MD and ab-into methods to investigate structure-property relationships, interfacial behavior, microstructural phenomena, diffusive transport and thermal transport properties from nanoscale simulations.

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