We use the recently developed reactive force field ReaxFF with molecular dynamics (MD) to study the role of voids on the initial chemical events in the high-energy material RDX under shock loading. We find that for strong shocks (particles velocity of 3 km/s) very small gaps (2 nm) lead to important over-heating (~ 1000 K). This over-heating facilitates chemical reactions and leads to a larger production of small molecules (such as NO2, NO, OH) than in perfect crystals shocked with the same strength. The chemical reactions occur after the void has collapsed and the ejected material re-compressed rather than when hot molecules are ejected out of the downstream surface

Goddard, William A., III

Strachan, Alejandro

van Duin, Adri C. T.

English

Alejandro Strachan

Crossref

INITIAL CHEMICAL EVENTS IN THE ENERGETIC MATERIAL RDX
UNDER SHOCK LOADING: ROLE OF DEFECTS
Alejandro Strachan
 
, Adri C. T. van Duin†William A. Goddard, III†

Theoretical Division, Los Alamos National Laboratory. Los Alamos, NM 87545
†Materials and Process Simulation Center, Beckman Institute (139-74), Caltech, Pasadena, CA 91125
Abstract. We use the recently developed reactive force field ReaxFF with molecular dynamics (MD) to
study the role of voids on the initial chemical events in the high-energy material RDX under shock loading.
We find that for strong shocks (particles velocity of 3 km/s) very small gaps (2 nm) lead to important over-
heating (  1000 K). This over-heating facilitates chemical reactions and leads to a larger production of small
molecules (such as NO2, NO, OH) than in perfect crystals shocked with the same strength. The chemical
reactions occur after the void has collapsed and the ejected material re-compressed rather than when hot
molecules are ejected out of the downstream surface.
INTRODUCTION
Understanding the initial chemical events in
condensed-phase high-energy materials under
shock or thermal loading is among the central prob-
lems in detonation theory. The coupling between
loading, induced mechanical response and complex
chemistry makes the study of such process very
challenging and there has been little progress in
establishing a molecular level understanding. Such
detailed understanding requires following the dy-
namics of reactions at a resolution in time and space
that can only be provided by atomistic Molecular
Dynamics (MD). Recently, Manaa et al. used MD
with atomic interactions obtained using Quantum
Mechanics (QM) to study the decomposition of
HMX at temperature and pressure close to the CJ
point [1]. They found that chemistry occurs very
rapidly: the time-scales for H2O production is  2
ps. Unfortunately QM methods are computationally
very intensive, making them impractical to study
shock waves (requiring simulations with at least
thousands of atoms). Such simulations for energetic
materials are now enabled by the recently developed
reactive force field ReaxFF [2], that accurately de-
scribes complex reactive processes while providing
the necessary computational efficiency to allow the
simulation of sufficiently large systems for suffi-
ciently long times. We have recently used ReaxFF
to study shock waves on RDX perfect crystals and
the initial chemical events induced by them [3]. We
found that for sufficiently strong shocks (particle
velocity vpart
 3km  s) RDX decomposes and reacts
to form a variety of small molecules. The time-scales
of the initial chemical processes are, again, very fast
(picoseconds).
Most solid energetic materials are inhomogeneous
and defective (polycrystalline, plastic bonded, with
voids) rather than perfect single crystals. The pres-
ence of such defects is believed to facilitate detona-
tion initiation under shock loading with defects help-
ing localize the energy in the shock. In this paper we
study the propagation of shock waves on defective
RDX crystals and the influence of such defects in the
initial chemical events using ReaxFF with MD.
ReaxFF REACTIVE FORCE FIELD
Traditional Force Fields describe energetics, struc-
tures and vibrations of molecular systems but are
unable to describe chemical reactions limiting their
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
FIGURE 1. Snapshots from ReaxFF MD shock simulation at different times for vpart=3 km/s and gap width 20 Å.
range of applicability. This problem was solved to a
large extent in recent years with the development of
reactive Force Fields based on the concept of partial
bond order [4, 5, 2]. Furthermore, MD simulations
using bond order potentials with simple chemistry
can provide valuable information about detonation
[5, 6].
The First Principles-based reactive Force Field
ReaxFF describes the total energy of an atomic sys-
tem with three terms [2]:
• Electrostatic interactions that allows charge
transfer between atoms in response to the
atomic environment; charges are calculated
self-consistently at every step during MD.
• Covalent interactions (bonds, angles, and tor-
sions) based on the concept of partial bond or-
der and calculated purely from atomic posi-
tions.
• Shielded van der Waals interactions calcu-
lated between all pairs of atoms.
It is important to stress that all three type of are de-
fined between every pair of atoms requiring no a
priori definition of molecule or connectivities. Each
atom is described with a unique set of potential func-
tions regardless of its environment.
In order to study RDX the original hydrocarbon
ReaxFF [2] was extended to include Oxygen and
Nitrogen. ReaxFF has been parameterized against a
large number of QM calculations [2, 3] including
bond breaking curves for each possible bond, angle
and torsion bending for each possible case as well
as several chemical reactions including RDX decom-
position mechanisms in the gas phase. ReaxFF ac-
curately describes the gas-phase chemistry of RDX
(energies and structures of intermediates as well as
transition states) as well as structural and mechani-
cal properties (lattice parameters, bulk modulus and
sound speed) [3].
MD SHOCK SIMULATIONS
In this paper we study the role of defects in shock
propagation in RDX using high velocity impact MD
simulations. We simulate the impact between two
two-dimensionally periodic slabs, one of which con-
tains a gap [see Figure 1 (a)] of length lgap. We im-
pose free boundary conditions in the direction of the
shock and periodic boundary conditions in the other
two directions. The gap simulates the center of an
ellipsoid void with two long axes perpendicular to
the shock direction [7, 8]. After thermalization, we
assigned the desired relative velocity (vimp) to each
slab on top of thermal velocities and followed the dy-
namics with constant energy MD. This arrangement
leads to a particle velocity vpart  0  5vimp. Figure 1
shows snapshots of the process at different times for
vpart  3km  s and lgap  20Å: we can see the ini-
tial shock propagation in perfect crystals (t=0.5 ps),
the molecules ejected from the upstream surface of
the gap expanding into the void (t = 0.95 ps), and the
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
ejected material colliding against the remainder of
the slab and re-compressing (t  1.5 ps). In this pa-
per we will focus on vpart  3km  s since for perfect
crystals it separates two different regimes: for shocks
with vpart
 3km  s a variety of small molecules are
formed while for lower velocities only small frac-
tions on NO2 are observed [3].
125 150 175 200 225 250 275
position (A)
-4000
-2000
0
2000
4000
lo
ca
l v
el
oc
ity
 (m
/s
)
125 150 175 200 225 250 275
position (A)
0
500
1000
1500
2000
2500
3000
lo
ca
l t
em
pe
ra
tu
re
 (K
)
t=0 ps
t=0.5 ps
t=1 ps
t=1.1 ps
t=1.25 ps
t=1.5 ps
t=0 ps t=0.5 ps
t=1 ps
t=1.1 ps
t=1.25 ps
t=1.5 ps
RDX
Vpart=3Km/s
Gap 20 A
RDX
Vpart=3Km/s
Gap 20 A
(a)
(b)
FIGURE 2. Particle velocity (a) and local temperature
(b) profiles at different times. Particle velocity 3 km/s, gap
width 20 Å.
Figure 2 shows profiles (along the shock direc-
tion) of the local particle velocity (top) and local
temperature (bottom). In order to obtain such profiles
we divide our system into 16 regions, each contain-
ing 8 RDX molecules. At t=0 ps (filled circles) we
see the imposed relative velocities and initial tem-
perature (10 K). At t=0.5 ps (open circles) the slabs
have collided and two symmetric shock waves are
propagating: the velocity of the shocked material is
zero and its temperature is between 1500 and 2000
K. From t=0.75 ps to t=1 ps (open diamonds) the
shock on the right slab has already reached the gap
and molecules near the upstream surface are accel-
erated into the void with local velocities relative to
the down-stream, un-shocked, portion of the right
slab reaching 2vpart . The ejecta from the free surface
hits the remainder of the slab at time 1<t<1.1 ps (see
filled squares) leading to high local temperatures (see
temperature profiles for t=1.1, 1.25 and 1.5 ps). We
can see from Figure 2 (b) that the local temperature
caused by the shock is much higher in the regions
surrounding the gap than that in the perfect slab.
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
time (ps)
0
500
1000
1500
2000
2500
lo
ca
l t
em
pe
ra
tu
re
 (K
)
Gap: 20 A
Gap: 10 A
Gap: 5A
RDX shocks
Vpart = 3 km/s
Perfect slabs
FIGURE 3. Time evolution of local temperature of the
last region (8 molecules) the shock goes through in the
downstream half of the right slab for lgap=5 Å(circles),
lgap=10 Å(squares), and lgap=20 Å(diamonds). We also
show that time evolution of the local temperature of the
corresponding (mirror) region in the perfect slab (black
lines)
In order to study the local heating caused by the in-
teraction of the shock wave and the gap as well as its
dependence on gap width we plot in Figure 3 the time
evolution of the local temperature of the last region
(8 molecules) the shock goes through in the down-
stream half of the right slab for lgap=5 Å(circles),
lgap=10 Å(squares), and lgap=20 Å(diamonds). We
also show that time evolution of the local tempera-
ture of the corresponding (mirror) region in the per-
fect (left) slab; the temperatures of the regions in the
perfect slab are independent of the gap width and
are displayed as black lines. We can see that the ini-
tial heating of the regions by the gap is similar to
that of the ones in the perfect slab but they reach
a lower temperature (independent of gap width) be-
cause some of the shock energy is used to acceler-
ate them into the void. When the ejected (expand-
ing) molecules collapse with the upstream part of
the right slab (at a time that depends on void width)
they re-compress and heat up to temperatures much
higher than that attained in the perfect slab. Holian
and collaborators proposed a model to explain this
overheating based on the re-compression of the evap-
orated ejecta and validated it with large-scale two di-
mensional MD simulations Lennard-Jones solids [7].
In agreement with their model we find that heating
increases with gap width.
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
125 150 175 200 225 250 275
position (A)
-2
0
2
4
6
8
10
12
po
pu
la
tio
n
RDX Vpart=3 km/s
Gap: 20 A
NO2 (circles)
NO (squares)
OH (dashed line, diamonds)
time = 2 ps
N2 (triangles)
FIGURE 4. Profile of the population of several species
for t=2 ps. RDX shock vpart  3km 	 s, gap width 20 Å.
Once we have seen local heating due to void-
shock interaction the main question is then: is the
overheating achieved with these void widths enough
to significantly facilitate the chemistry ?. Figure 4
shows the profiles (along the shock propagation di-
rection) of the population of four small molecules
[NO2 (circles), NO (squares), OH (dashed lines with
diamonds) and N2 (triangles)] at time t=2 ps. Black
and gray circles bellow the profiles show the atomic
positions at t=2 ps differentiating the atoms belong-
ing to the left slab and to both portions of the defec-
tive slab. It is clear from figure 4 that the population
profiles are very asymmetric with a larger quantity of
products formed in the defective crystal. The quanti-
ties of NO and OH produced on the defective half
are comparable to those obtained in perfect crystals
at a higher impact velocity: vpart  4km  s [3]. Much
larger simulations simulations will be necessary to
establish if the enhancement of the initial chemistry
observed here is enough to generate a self-sustained
detonation and consequently increase the sensitivity
of the material or if instead it will die out.
CONCLUSIONS
First Principles-based atomistic modeling of shock
loading of high-energy nitramines have become fea-
sible due to the development of the reactive force
field ReaxFF. ReaxFF with MD allows full-physics,
full-chemistry simulations of shock waves propa-
gating in perfect and defective HE crystals and the
chemical reactions they induced. It is important to
stress that no approximation is made as to what type
of chemical reactions are allowed or what type of
molecules can be formed.
We found that even small gaps (few nanometers)
can lead to a local increase in temperature of over
1000 K on top of the shock heating. We have shown
that this heating enhances and facilitates the ini-
tial chemical reactions. Thus, even small defects can
play a key role in reducing the detonation threshold.
Future work we will focus on quantifying the de-
crease in detonation threshold as a function of void
size.
ACKNOWLEDGMENTS
A.S. thanks B. L. Holian, T. C. Germann and E. M.
Kober for helpful discussions. Work at Los Alamos
was supported by the ASC Materials and Physics
Modeling program. Work at Caltech was supported
by DOE-ASC-ASAP and ONR (Judah Goldwasser).
REFERENCES
1. Manaa, M. R., Fried, L. E., Melius, C. F., Elstner, M.,
and Frauenheim, T., J. Phys. Chem. A, p. 9024 (2002).
2. van Duin, A. C. T., Dasgupta, S., Lorant, F., and
Goddard III, W. A., J. Phys. Chem. A, 105, 9396 –
9409 (2001).
3. Strachan, A., van Duin, A. C. T., Chakraborty, D.,
Dasgupta, S., and Goddard III, W. A., Phys. Rev. Lett.
(In press 2003).
4. Brenner, D., Shenderova, O., Harrison, J., Stuart, S.,
Ni, B., and Sinnott, S., J. Phys.: Cond. Matt., 14, 783
(2002).
5. Brenner, D., Robertson, D., Elert, M., and White, C.,
Phys. Rev. Lett., 70, 2174 (1993).
6. Rice, B. M., Mattson, W., and Trevino, S., Phys. Rev.
E., 57, 5106 (1998).
7. Holian, B. L., Germann, T. C., Maillet, J. B., and
White, C. T., Phys. Rev. Lett., 89, 285501 (2002).
8. Germann, T. C., Holian, B. L., Lomdahl, P. S.,
Heim, A. J., Gronbech-Jensen, N., and Maillet, J. B.,
Proceedings of the 12th Symposium (International)
on Detonation (2002), URL http://www.sainc.
com/detsymp/technicalProgram.htm.
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp


Initial Chemical Events in the Energetic Material RDX under Shock Loading: Role of Defects

INITIAL CHEMICAL EVENTS IN THE ENERGETIC MATERIAL RDX
UNDER SHOCK LOADING: ROLE OF DEFECTS
Alejandro Strachan
 
, Adri C. T. van DuinWilliam A. Goddard, III

Theoretical Division, Los Alamos National Laboratory. Los Alamos, NM 87545
†Materials and Process Simulation Center, Beckman Institute (139-74), Caltech, Pasadena, CA 91125
Abstract. We use the recently developed reactive force eld ReaxFF with molecular dynamics (MD) to
study the role of voids on the initial chemical events in the high-energy material RDX under shock loading.
We nd that for strong shocks (particles velocity of 3 km/s) very small gaps (2 nm) lead to important over-
heating (  1000 K). This over-heating facilitates chemical reactions and leads to a larger production of small
molecules (such as NO2, NO, OH) than in perfect crystals shocked with the same strength. The chemical
reactions occur after the void has collapsed and the ejected material re-compressed rather than when hot
molecules are ejected out of the downstream surface.
INTRODUCTION
Understanding the initial chemical events in
condensed-phase high-energy materials under
shock or thermal loading is among the central prob-
lems in detonation theory. The coupling between
loading, induced mechanical response and complex
chemistry makes the study of such process very
challenging and there has been little progress in
establishing a molecular level understanding. Such
detailed understanding requires following the dy-
namics of reactions at a resolution in time and space
that can only be provided by atomistic Molecular
Dynamics (MD). Recently, Manaa et al. used MD
with atomic interactions obtained using Quantum
Mechanics (QM) to study the decomposition of
HMX at temperature and pressure close to the CJ
point [1]. They found that chemistry occurs very
rapidly: the time-scales for H2O production is  2
ps. Unfortunately QM methods are computationally
very intensive, making them impractical to study
shock waves (requiring simulations with at least
thousands of atoms). Such simulations for energetic
materials are now enabled by the recently developed
reactive force eld ReaxFF [2], that accurately de-
scribes complex reactive processes while providing
the necessary computational efciency to allow the
simulation of sufciently large systems for suf-
ciently long times. We have recently used ReaxFF
to study shock waves on RDX perfect crystals and
the initial chemical events induced by them [3]. We
found that for sufciently strong shocks (particle
velocity vpart
 3km  s) RDX decomposes and reacts
to form a variety of small molecules. The time-scales
of the initial chemical processes are, again, very fast
(picoseconds).
Most solid energetic materials are inhomogeneous
and defective (polycrystalline, plastic bonded, with
voids) rather than perfect single crystals. The pres-
ence of such defects is believed to facilitate detona-
tion initiation under shock loading with defects help-
ing localize the energy in the shock. In this paper we
study the propagation of shock waves on defective
RDX crystals and the inuence of such defects in the
initial chemical events using ReaxFF with MD.
ReaxFF REACTIVE FORCE FIELD
Traditional Force Fields describe energetics, struc-
tures and vibrations of molecular systems but are
unable to describe chemical reactions limiting their
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
FIGURE 1. Snapshots from ReaxFF MD shock simulation at different times for vpart=3 km/s and gap width 20 Å.
range of applicability. This problem was solved to a
large extent in recent years with the development of
reactive Force Fields based on the concept of partial
bond order [4, 5, 2]. Furthermore, MD simulations
using bond order potentials with simple chemistry
can provide valuable information about detonation
[5, 6].
The First Principles-based reactive Force Field
ReaxFF describes the total energy of an atomic sys-
tem with three terms [2]:
• Electrostatic interactions that allows charge
transfer between atoms in response to the
atomic environment; charges are calculated
self-consistently at every step during MD.
• Covalent interactions (bonds, angles, and tor-
sions) based on the concept of partial bond or-
der and calculated purely from atomic posi-
tions.
• Shielded van der Waals interactions calcu-
lated between all pairs of atoms.
It is important to stress that all three type of are de-
ned between every pair of atoms requiring no a
priori denition of molecule or connectivities. Each
atom is described with a unique set of potential func-
tions regardless of its environment.
In order to study RDX the original hydrocarbon
ReaxFF [2] was extended to include Oxygen and
Nitrogen. ReaxFF has been parameterized against a
large number of QM calculations [2, 3] including
bond breaking curves for each possible bond, angle
and torsion bending for each possible case as well
as several chemical reactions including RDX decom-
position mechanisms in the gas phase. ReaxFF ac-
curately describes the gas-phase chemistry of RDX
(energies and structures of intermediates as well as
transition states) as well as structural and mechani-
cal properties (lattice parameters, bulk modulus and
sound speed) [3].
MD SHOCK SIMULATIONS
In this paper we study the role of defects in shock
propagation in RDX using high velocity impact MD
simulations. We simulate the impact between two
two-dimensionally periodic slabs, one of which con-
tains a gap [see Figure 1 (a)] of length lgap. We im-
pose free boundary conditions in the direction of the
shock and periodic boundary conditions in the other
two directions. The gap simulates the center of an
ellipsoid void with two long axes perpendicular to
the shock direction [7, 8]. After thermalization, we
assigned the desired relative velocity (vimp) to each
slab on top of thermal velocities and followed the dy-
namics with constant energy MD. This arrangement
leads to a particle velocity vpart  0  5vimp. Figure 1
shows snapshots of the process at different times for
vpart  3km  s and lgap  20¯: we can see the ini-
tial shock propagation in perfect crystals (t=0.5 ps),
the molecules ejected from the upstream surface of
the gap expanding into the void (t = 0.95 ps), and the
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
ejected material colliding against the remainder of
the slab and re-compressing (t  1.5 ps). In this pa-
per we will focus on vpart  3km  s since for perfect
crystals it separates two different regimes: for shocks
with vpart
 3km  s a variety of small molecules are
formed while for lower velocities only small frac-
tions on NO2 are observed [3].
125 150 175 200 225 250 275
position (A)
-4000
-2000
0
2000
4000
lo
ca
l v
el
oc
ity
 (m
/s
)
125 150 175 200 225 250 275
position (A)
0
500
1000
1500
2000
2500
3000
lo
ca
l t
em
pe
ra
tu
re
 (K
)
t=0 ps
t=0.5 ps
t=1 ps
t=1.1 ps
t=1.25 ps
t=1.5 ps
t=0 ps t=0.5 ps
t=1 ps
t=1.1 ps
t=1.25 ps
t=1.5 ps
RDX
Vpart=3Km/s
Gap 20 A
RDX
Vpart=3Km/s
Gap 20 A
(a)
(b)
FIGURE 2. Particle velocity (a) and local temperature
(b) profiles at different times. Particle velocity 3 km/s, gap
width 20 Å.
Figure 2 shows proles (along the shock direc-
tion) of the local particle velocity (top) and local
temperature (bottom). In order to obtain such proles
we divide our system into 16 regions, each contain-
ing 8 RDX molecules. At t=0 ps (lled circles) we
see the imposed relative velocities and initial tem-
perature (10 K). At t=0.5 ps (open circles) the slabs
have collided and two symmetric shock waves are
propagating: the velocity of the shocked material is
zero and its temperature is between 1500 and 2000
K. From t=0.75 ps to t=1 ps (open diamonds) the
shock on the right slab has already reached the gap
and molecules near the upstream surface are accel-
erated into the void with local velocities relative to
the down-stream, un-shocked, portion of the right
slab reaching 2vpart . The ejecta from the free surface
hits the remainder of the slab at time 1<t<1.1 ps (see
lled squares) leading to high local temperatures (see
temperature proles for t=1.1, 1.25 and 1.5 ps). We
can see from Figure 2 (b) that the local temperature
caused by the shock is much higher in the regions
surrounding the gap than that in the perfect slab.
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
time (ps)
0
500
1000
1500
2000
2500
lo
ca
l t
em
pe
ra
tu
re
 (K
)
Gap: 20 A
Gap: 10 A
Gap: 5A
RDX shocks
Vpart = 3 km/s
Perfect slabs
FIGURE 3. Time evolution of local temperature of the
last region (8 molecules) the shock goes through in the
downstream half of the right slab for lgap=5 Å(circles),
lgap=10 Å(squares), and lgap=20 Å(diamonds). We also
show that time evolution of the local temperature of the
corresponding (mirror) region in the perfect slab (black
lines)
In order to study the local heating caused by the in-
teraction of the shock wave and the gap as well as its
dependence on gap width we plot in Figure 3 the time
evolution of the local temperature of the last region
(8 molecules) the shock goes through in the down-
stream half of the right slab for lgap=5 ¯(circles),
lgap=10 ¯(squares), and lgap=20 ¯(diamonds). We
also show that time evolution of the local tempera-
ture of the corresponding (mirror) region in the per-
fect (left) slab; the temperatures of the regions in the
perfect slab are independent of the gap width and
are displayed as black lines. We can see that the ini-
tial heating of the regions by the gap is similar to
that of the ones in the perfect slab but they reach
a lower temperature (independent of gap width) be-
cause some of the shock energy is used to acceler-
ate them into the void. When the ejected (expand-
ing) molecules collapse with the upstream part of
the right slab (at a time that depends on void width)
they re-compress and heat up to temperatures much
higher than that attained in the perfect slab. Holian
and collaborators proposed a model to explain this
overheating based on the re-compression of the evap-
orated ejecta and validated it with large-scale two di-
mensional MD simulations Lennard-Jones solids [7].
In agreement with their model we nd that heating
increases with gap width.
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
125 150 175 200 225 250 275
position (A)
-2
0
2
4
6
8
10
12
po
pu
la
tio
n
RDX Vpart=3 km/s
Gap: 20 A
NO2 (circles)
NO (squares)
OH (dashed line, diamonds)
time = 2 ps
N2 (triangles)
FIGURE 4. Profile of the population of several species
for t=2 ps. RDX shock vpart  3km 	 s, gap width 20 Å.
Once we have seen local heating due to void-
shock interaction the main question is then: is the
overheating achieved with these void widths enough
to signicantly facilitate the chemistry ?. Figure 4
shows the proles (along the shock propagation di-
rection) of the population of four small molecules
[NO2 (circles), NO (squares), OH (dashed lines with
diamonds) and N2 (triangles)] at time t=2 ps. Black
and gray circles bellow the proles show the atomic
positions at t=2 ps differentiating the atoms belong-
ing to the left slab and to both portions of the defec-
tive slab. It is clear from gure 4 that the population
proles are very asymmetric with a larger quantity of
products formed in the defective crystal. The quanti-
ties of NO and OH produced on the defective half
are comparable to those obtained in perfect crystals
at a higher impact velocity: vpart  4km  s [3]. Much
larger simulations simulations will be necessary to
establish if the enhancement of the initial chemistry
observed here is enough to generate a self-sustained
detonation and consequently increase the sensitivity
of the material or if instead it will die out.
CONCLUSIONS
First Principles-based atomistic modeling of shock
loading of high-energy nitramines have become fea-
sible due to the development of the reactive force
eld ReaxFF. ReaxFF with MD allows full-physics,
full-chemistry simulations of shock waves propa-
gating in perfect and defective HE crystals and the
chemical reactions they induced. It is important to
stress that no approximation is made as to what type
of chemical reactions are allowed or what type of
molecules can be formed.
We found that even small gaps (few nanometers)
can lead to a local increase in temperature of over
1000 K on top of the shock heating. We have shown
that this heating enhances and facilitates the ini-
tial chemical reactions. Thus, even small defects can
play a key role in reducing the detonation threshold.
Future work we will focus on quantifying the de-
crease in detonation threshold as a function of void
size.
ACKNOWLEDGMENTS
A.S. thanks B. L. Holian, T. C. Germann and E. M.
Kober for helpful discussions. Work at Los Alamos
was supported by the ASC Materials and Physics
Modeling program. Work at Caltech was supported
by DOE-ASC-ASAP and ONR (Judah Goldwasser).
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Initial Chemical Events in the Energetic Material RDX under Shock Loading: Role of Defects

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