Structure and Density Predictions for

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Structure and Density Predictions for Energetic
Materials
Zuyue Du, Sayta Prasad, Ed Wells and Herman L.
Ammon Department of Chemistry
Biochemistry, University of Maryland College
Park, MD 20742 Picatinny Arsenal, NJ Oct. 27,
2004
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Density --gt "primary physical parameter in
detonation performance"
Cyclic nitramines, (CH2NNO2)n units
exptl structure
prediction rVA mass / S(atom/group
volumes)
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r problems with constitutional isomers.
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Crystal Structure Prediction
ab initio geometry optimization usually G03,
b3lyp/631g
model
Mimic exptl crystal structures for triclinic -gt
orthorhombic sp grps Use exptl coordn geom
patterns Create hypothetical crystal
structures for 29 coordination
geometries 6,900 structures/coordn geom
MOLPAK
400-900 highest r each geom
Optimize unit cell parameters, model orientation
position by lattice E minimization WMIN
atom-centered charge electrostatics DMAREL
distributed multipole electrostatics
refinement
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MOLPAK coverage all triclinic to Z 8
orthorhombic space groups For C-H-N-O-F
molecules, space group frequencies are
Sp Grp N
Sp Grp N
MOLPAK coordination geometries S29 99
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WMIN LE potential
Optimize crystal structure Adjust unit cell,
model position orientation
A, B, C empirical coefficients Aij
(Aatom_type_i Aatom_type_j)1/2 Currently have
69 atoms types Eg, C-NO2, N-NO2, cubane-NO2,
N-NO2 R3N, C(O)NR2
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Crystal structure prediction successes and
problems
X-ray model
29400-900/geom -gt lattice refinement LE calcns
Examples.. (CH3)2NNO2 TNAZ RDX e-CL20
FOX7 2, 0.4
In 80 smallest LE -gt exptl structure
In 20 exptl structure in top 6 lowest LE
solutions
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Structure prediction example, e-CL20 from
B3lyp/631g model
r E Space r E
Space group
group 1 1.996 -43.56 P21/c 16 1.856
-36.46 C2/c 2 1.966 -42.28 P212121 17
1.870 -35.92 P21
3 1.965 -42.20 P212121 18
1.777 -35.85 P21
4 1.898 -39.63 P21/c
19 1.856 -35.56 Cc
5 1.898 -39.63 P21/c 20
1.804 -35.50 Pbcn
6 1.838 -38.07 C2/c 21
1.783 -35.32 Pna21
7 1.851 -37.99 Pbca 22
1.817 -34.63 Pbcn 8
1.875 -37.81 Pca21 23 1.721 -33.68 P21212
9 1.901 -37.68 P21/c
24 1.734 -33.57 C2
10 1.832 -37.34 C2/c 25
1.690 -32.56 Pna21
11 1.802 -37.24 Pbca 26
1.799 -31.96 Pca21
12 1.877 -37.18 P-1 27 1.706 -31.34 P1
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1.877 -37.18 P-1 28 1.656 -31.04 P21212
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1.835 -37.11 Pna21 29 1.693 -29.85 P21212
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1.810 -36.54 P21/c
Cell Parameters a b c b
X-ray 8.852 Å 12.556 13.386
106.82o Predicted D () 1.55 -0.53 0.98
-0.65
robsd 2.043 g/cc
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Structure prediction -gt new molecule evaluation
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ROTPAKpack adjust conformation, minimize
Etotal Criterion Etotal Einter Eintra
Ethresh
closest approach
oriented moving molecule
oriented molecule at origin
move together until Etotal Ethresh
Ethresh 0.5 kcal/mol for line
alter conformation calc new Etotal
lt Ethresh
gt Ethresh
no further improvement in Etotal
new orientation
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ROTPAK examples
Model (G03), ROTPAK/WMIN and X-ray torsion angle
comparison
G03 RPK Xray G03 RPK Xray
G03 RPK Xray 1 29.4º-gt 26.0 20.9 1
-5.4º-gt -5.2 -6.0 1 11.6º-gt 11.6 9.6 2
0.0 -gt 11.4 19.4 2 167.4 -gt 173.6 179.3 2
-76.6 -gt -1.6 -5.0 3 21.6 -gt 10.3 7.2 3
-108.5 -gt -30.6 -32.6 4 1.3 -gt 1.8 13.5 5
30.0 -gt -17.7 -16.4 6 -1.5 -gt 30.3 25.4 7
38.3 -gt -12.9 -9.2
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ROTPAK example
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2
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X-ray
B3lyp/631g model
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X-ray
G03 ROTPAK WMIN
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ROTPAK exampleRDX bond bending
G03 B3lyp/631g model
X-ray model
ROTPAK WMIN
bend 20o N-NO2 twist
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RDX X-ray and ROTPAK-modified overlays
RDX space group Pbca unit
cell comparison
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Some ROTPAK challenges
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• MOLPAK, the old and the new
• MOLPAK-1
• Uses pre-established coordn geoms (CN 14)
• Coordn geom sub-programs are hand-coded
• Some structures dont fit the rules
• Structures built with repulsion-only potential
• MOLPAK-2
• Build structures from crystal space group
  symmetry
• One program does all symmetries
• Structures built with 3-term potential
• Special features more easily handled, eg
  H-bonding,
• molecule-solvent complexes, ionic materials

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MOLPAK-2 flowchart for P21/c
Criteria van der Waals radii repulsion energy
total energy
Determine length of an axis -gt make line of C1 or
Ci images assume line is a/c or b axis
Make rectangular box around origin, 0.5 Å spacing
C1/Ci image at origin each grid point in
succession
Pre-LE calcn criteria van der Waals
radii crystal density range
For each grid, fill unit cell calculate lattice
energy
Symmetry elements inversion center, 2-fold
screw axis, glide plane
order surviving unit cells on basis of LE
Lowest LE -gt correct structure
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MOLPAK-2 examples
MOLPAK-1 failed -gt no axial repeat molecules in
coodn sphere
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• Continuing and future work and goals
• Global atom type parameterization for both
• WMIN and DMAREL -gt new atom types
• potential function cross-terms
• anisotropic potential coefficients
• Identification of the best/correct structure -gt
• LEs and rs used currently
• patterns of intermolecular contacts?
• crystal habits/crystal face Es?
• ROTPAK conformational flexibility -gt
• continue development -gt
• focus on intramolecular E evaluation
• multi-bond flexibility
• MOLPAK-2 -gt continue development -gt
• handle all space groups

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• Extend to ionics (eg ADN), H-bonding,
• high N compounds
• New lattice energy refinement code
• (WMIN replacement) -gt modern fortran
• analytical derivatives
• automate use of individual atom types/
• cross-terms in LE potential
• facilitate conformational refinement
• Sensitivity -gt density of states
• impact/shock friction weakest bond
  lattice E
• steric hindrance to sheer
• CHSSI Super parallel MOLPAK/WMIN
• w/ B. Rice W. Mattson

ARL, Picatinny Indian Head B. Chapman K. Baum H.
Shechter P. Eaton J. Bottaro
thanks