1. Physical background
Explosives containing C, H, N, O produce mixture of N2, H20, C02
at high temperatures(10005000K) and high pressures(1500GPa).
In [1], it was predicted that under such extreme condition N2/H20 mixture
mixture might exhibit phase separation behind detonation waves. This prediction
was investigated by Prof. Koshi, M. and Matsui, H. who confirmed
that under certain conditions on the temperature and density the
phase separation indeed occurs. This investigation has been done
using molecular dynamics program NVT3, originally written by Prof. Koshi
in FORTRAN 77. The size of the system was N=256 molecules (128 H20 + 128
N2) and the temperature T=2000 K.
2. Description of NVT3 molecular dynamics program
NVT3 program computes molecular dynamics for the binary mixture
for NVT or NVE ensembles. The force/energy computation uses modified
Buckingham (exp6) potential. The motion integration is done using
leapfrog method. During simulation run program outputs temperature,
pressure, total energy of the system plus some additional parameters.
To analyze the structure of the binary mixture, program also outputs
pair correlation function data. 3. Acceleration of MD program NVT3
We converted NVT3 program, kindly provided to us by Prof. Koshi,
first to C and then to CUDA. Then, to observe realtime molecular
dynamics simulation we added particle rendering capability and disabled
computation of some system parameters not necessary for the MD computation.
As a result, we observed 37x to 60x acceleration of MD simulation compared
to allCPU version of this program. 4. Results.
Below are the actual images of the equimolar binary system at
the nearinitial state (step 10, t=10 femtoseconds) and at the phase
separated state (step 10400, t=10.4 picoseconds). The number
of molecules in this simulation is N = 13500, the temperature is T=2500K
and the density is ρ = 2.3867 g / cm^{3}. This simulation was
run for 20000 timesteps, which corresponds to 20 picoseconds.
On Intel (R) Core(TM) i7 machine with GeForce GT480 GPU, the computational
time of the entire simulation was 12 minutes.

Figure 1: Nearinitial state (10ps)
of the H20/N2 system. No phase separation can be observed. 

Figure 2: Step 10400 (10.4 ps): Phase
separation occurred: one can clearly see the clustering of the water
molecules (blue) and the nitrogen molecules (white). 
Download movie of the entire simulation, WM9 compressed:
High Quality (60MB) Medium Quality (30MB) Low Quality (5MB)
5. Possible applications
The program can be used to perform MD simulations of the
binary molecular mixtures interacting by exp6 potential. The size
of the system can be large, of the order of tens of thousands.
It is also possible to modify the program to compute molecular dynamics of 3 or more components, interacting by either exp6 or by some other potential.
6. References
[1] F.H. Ree,
”Supercritical fluid phase separations: Implications for detonation properties of condensed explosives” ,
J. Chem. Phys.,84, 5845 (1986)
[2] Koshi, M., Matsui, H., “Molecular Dynamics Study of High Temperature
PhaseSeparation in H2O/N2 mixture with exp6 interactions”, Molecular Simulation,
1994, Vol. 12(36), pp. 227239.