In solids and molecules, atoms vibrate about their respective
equilibrium positions, and this thermal motion can be understood as a
superposition of the structure’s normal modes, which are collective
movements of atoms vibrating at certain frequencies. These vibrational
modes play important roles in a variety of material properties and
physical phenomena such as chemical reactions, mass/ion diffusivity,
phase transitions, thermal conductivity, electrical conductivity, and
any property which is affected by atomic motion. It is therefore
important to study the dynamics and energy transfer processes of
normal modes in a variety of systems, so that we may better understand
or engineer a wide variety of phenomena.
Here I introduce a computational framework for studying vibrational
modes in any general system by calculating various mode properties
such as anharmonic coupling constants. With this information, we may
simulate time-dependent energy transfer processes among vibrational
modes, and study their heat transfer in any general system of atoms. I
show example calculations and simulations to elucidate the physical
mechanisms of heat transfer in crystalline, amorphous, and alloy
materials, along with mechanisms of interfacial heat transfer. This
framework is packaged into an open-source program called ModeCode
which uses the LAMMPS C++ library to generally calculate vibrational
modes in any system modeled by any potential in LAMMPS. ModeCode is
massively parallel to support the scalable calculation of vibrational
modes in systems with ~100,000+ atoms, allowing us to computationally
study vibrational modes in realistic systems of interest.
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