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Structure and Density Predictions for Energetic Materials

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. pt grp. r g/cc. r VA ***. RDX. C s. 1.806*.

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Structure and Density Predictions for Energetic Materials

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  1. 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

  2. pt grp r g/cc rVA*** RDX Cs 1.806* 1.838 a-HMX C2 1.839* 1.847 b-HMX Ci 1.902* “ d-HMX C2v 1.759* “ 4-ring ~Ci 1.668** 1.829 10-ring C1 1.750** 1.810 Density --> "primary physical parameter in detonation performance" Cyclic nitramines, (CH2NNO2)n units *exptl **structure prediction *** rVA = mass / S(atom/group volumes)

  3. r problems withconstitutional isomers….

  4. Crystal Structure Prediction… ab initio geometry optimization; usually G03, b3lyp/631g* model Mimic exptl crystal structures for triclinic -> 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

  5. 1 P1 [1] 58 0.4 21 C222 1 0.01 2 P-1 [2] 1747 12.6 23 I222 2 0.01 4 P21 [2] 1507 10.9 29 Pca21 [2] 137 1.0 5 C2 [1] 125 0.9 33 Pna21 [3] 253 1.8 7 Pc 46 0.3 34 Pnn2 2 0.01 9 Cc [1] 135 1.0 41 Aba2 10 0.1 13 P2/c 12 0.1 45 Iba2 9 0.1 14 P21/c [5] 5157 37.2 52 Pncn 1 0.01 15 C2/c [3] 557 4.0 56 Pccn 35 0.3 18 P21212 [3] 57 0.4 60 Pbcn [2] 62 0.4 19 P212121 [2] 3210 23.2 61 Pbca [2] 714 5.2 20 C2221 14 0.1 MOLPAK coverage… all triclinic to Z = 8 orthorhombicspace groups For C-H-N-O-F molecules, space group frequencies are… # Sp Grp N % # Sp Grp N % MOLPAK coordination geometries = S[29] ~ 99%

  6. 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

  7. Crystal structure prediction… successes and problems… X-ray model 29*400-900/geom -> lattice refinement & LE calcns Examples.. (CH3)2NNO2 * TNAZ * RDX * e-CL20 * FOX7 # 2, +0.4 In 80% smallest LE -> exptl structure In 20% exptl structure in top 6 lowest LE solutions

  8. 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 13 1.877 -37.18 P-1 28 1.656 -31.04 P21212 14 1.835 -37.11 Pna21 29 1.693 -29.85 P21212 15 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

  9. Structure prediction -> new molecule evaluation…

  10. ROTPAK…pack & 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 < Ethresh > Ethresh no further improvement in Etotal new orientation

  11. ROTPAK examples… Model (G03), ROTPAK/WMIN and X-ray torsion angle comparison… # G03 RPK Xray # G03 RPK Xray # G03 RPK Xray 1 29.4º-> 26.0 20.9 1 -5.4º-> -5.2 -6.0 1 11.6º-> 11.6 9.6 2 0.0 -> 11.4 19.4 2 167.4 -> 173.6 179.3 2 -76.6 -> -1.6 -5.0 3 21.6 -> 10.3 7.2 3 -108.5 -> -30.6 -32.6 4 1.3 -> 1.8 13.5 5 30.0 -> -17.7 -16.4 6 -1.5 -> 30.3 25.4 7 38.3 -> -12.9 -9.2

  12. X-ray B3… RPK/ WMIN 1 41.4o 0.0o 45.0o 2 51.9 47.1 48.6 3 22.9 24.6 29.0 ROTPAK example… 3 2 1 X-ray B3lyp/631g* model

  13. a 12.57 Å 6.8% b 4.85 1.4 c 19.67 1.8 b 119.9o 2.9 r 1.53 -6.3 X-ray G03 + ROTPAK + WMIN

  14. ROTPAK example…RDX bond bending… G03 B3lyp/631g* model X-ray model ROTPAK + WMIN bend ~20o + N-NO2 twist

  15. dimension X-ray ROTPAK- X-ray (%) a (Å) 13.182 1.2 b 11.574 0.7 c 10.709 -0.6 r(g/cc) 1.806 -1.3 RDX X-ray and ROTPAK-modified overlays… RDX space group: Pbca unit cell comparison

  16. Some ROTPAK challenges…

  17. MOLPAK, the old and the new… • MOLPAK-1 • Uses pre-established coordn geoms (CN = 14) • Coordn geom sub-programs are hand-coded • Some structures don’t 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

  18. MOLPAK-2 flowchart for P21/c… Criteria: van der Waals radii repulsion energy total energy Determine length of an axis -> 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 -> correct structure

  19. MOLPAK-2 examples… MOLPAK-1 failed -> no axial repeat molecules in coodn sphere

  20. Continuing and future work and goals… • Global atom type parameterization for both • WMIN and DMAREL ->new atom types • potential function cross-terms • anisotropic potential coefficients • Identification of the best/correct structure -> • LE’s and r‘s used currently • patterns of intermolecular contacts? • crystal habits/crystal face E’s? • ROTPAK & conformational flexibility -> • continue development -> • focus on intramolecular E evaluation & • multi-bond flexibility • MOLPAK-2 -> continue development -> • handle all space groups • extensive testing

  21. Extend to ionics (eg ADN), H-bonding, • high N compounds • New lattice energy refinement code • (WMIN replacement) -> modern fortran • analytical derivatives • automate use of individual atom types/ • cross-terms in LE potential • facilitate conformational refinement • Sensitivity -> 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

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