Simulations of Dense Plasmas
Lee Collins (T-4) and Joel Kress (T-12)


 A dense plasma forms a peculiar media, exhibiting features of a solid, liquid, and ionized 
gas. The density approaches or exceeds that of solid matter (1022-1026 atoms/cm3) yet the 
media behaves as a freely-flowing fluid.  Due to the proximity of the atoms or to the 
temperature, the system also ionizes, yielding its plasma nature. Dense plasmas play a crucial 
role in such diverse regimes as white dwarf stars, planetary interiors, interial confinement 
fusion, pulse-power generators, laser-matter interfaces, sonoluminescence, and certain liquid 
metals.  Plasmas are generally considered classical entities. However, for this case, the very 
close packing of the atoms so that their electronic wavefunctions strongly overlap forces a 
quantum mechanical treatment. The media resembles a quantal "fluid," where all species must 
be treated democratically.

To model such a complex media requires very sophisticated techniques so that all possible 
mechanisms such as molecular bonding and dissociation, cluster formation, ionization-
recombination, and attachment-detachment receive treatment on an equal footing.  We have 
employed quantum molecular dynamics (QMD) techniques to effect this simulation. The 
electronic component of a composite system of atoms is treated within a periodically-
replicated reference cell by a finite-temperature density functional method in the local 
density approximation, and the nuclear motion by classical mechanics.  We have also 
investigated more approximate electronic structure methods such as tight-binding and various 
zero-differential-overlap forms. An example of a typical QMD simulation for a system heated 
above the melting point appears in Figure 1. The system begins in a solid state of high 
symmetry (blue), passes through a molecular stage (yellow), and ends in a disordered 
collection of atoms (red).

We have focused on hydrogen (H) compressed to over ten times its solid density at 
temperatures up to hundreds of thousands of degrees Kelvin. At very low temperatures, we 
find a complex fluid, permeated with long chains or filaments of atoms. Increasing the 
temperature yields an insulating diatomic molecular fluid and eventually a purely atomic, 
highly conducting media. Even at the elevated temperatures, transient associations of atoms 
form that profoundly affect both radiative and transport properties.

We also have examined liquid alkali metals, impurities such as argon in H plasmas, and the 
insulator to metal transition in rare gases. The former exhibits even more complex behavior 
than H at low temperatures. For example, an expanded sodium (Na) fluid fills with bound 
clusters of between two and eight atoms.  Figure 2 illustrates this effect for a single snapshot 
taken during a long trajectory simulation, displaying the atoms (red) and their associated 
bonds (white).

Quantum molecular dynamics provides a powerful tool for exploring intricate properties of 
matter at conditions of high temperatures or pressures.

Collaborators include I. Kwon (Georgia); N. Troullier (IBM); and O. 
Pfaffenzeller and D. Hohl (Juelich, Germany).