Structure and Density Predictions forStructure and Density Predictions forEnergetic MaterialsEnergetic Materials
Zuyue Du, Sayta Prasad, Ed Wells and Herman L. Ammon
Department of Chemistry & Biochemistry,University of MarylandCollege Park, MD 20742
Picatinny Arsenal, NJOct. 27, 2004
computer modeling &
simulations
quantitative estimation
key properties & characteristics... conformation stability/reactivity Hf crystal structure/habit microstructure detonation behavior sensitivity
elemental composition&
molecular structure+
Identification,Design andDevelopment
of new energeticmaterials
Cyclic nitramines, (CH2NNO2)n units
1.810
1.829
“
“
1.847
1.838
VA***
1.750**
1.668**
1.759*
1.902*
1.839*
1.806*
g/cc
C1
~Ci
C2v
Ci
C2
Cs
pt grp
10-ring
4-ring
-HMX
-HMX
-HMX
RDX
*exptl **structure prediction
*** VA = mass / (atom/group volumes)
N
N
NNO2
NO2
O2N
NN
NN NO2
O2N
O2N
NO2
RDX
HMX
NN
NN
N
O2N
NO2
NO2O2N
O2N 10N
N
O2N
NO2
4
DensityDensity --> "primary physical parameter in detonation performance"
problems withproblems with constitutional isomers….constitutional isomers….
O2N NO2 O2N
O2NNO2 NO2
exptl : 1.573 1.569 1.651 g/cc
the solutionsolid state
structure prediction
many variables:atom types,
structure/conformation,packing efficiency
MOLPAKROTPAKMOLPAK-2
Crystal Structure Prediction…Crystal Structure Prediction…
model
MOLPAK
refinement
ab initio geometry optimization;usually G03, b3lyp/631g*
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
Optimize unit cell parameters, model orientation & position by lattice E minimization;
WMIN: atom-centered charge electrostaticsDMAREL: distributed multipole electrostatics
400-900highest each geom
0.114C222120
5.2714Pbca [2]6123.23210P212121 [2]19
0.462Pbcn [2]600.457P21212 [3]18
0.335Pccn564.0557C2/c [3]15
0.011Pncn5237.25157P21/c [5]14
0.19Iba2450.112P2/c13
0.110Aba2411.0135Cc [1]9
0.012Pnn2340.346Pc7
1.8253Pna21 [3]330.9125C2 [1]5
1.0137Pca21 [2]2910.91507P21 [2]4
0.012I2222312.61747P-1 [2]2
0.011C222210.458P1 [1]1
MOLPAK coverageMOLPAK 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 = [29] ~ 99%
WMIN LE potential…WMIN LE potential…
Optimize crystal structure…Adjust unit cell, model position & orientation
LE = [QiQj*Rij-1 - Aij*Rij
-6 + Bij*exp-(Rij(Ci+Cj))]
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
N N
O
Calcd vs. X-ray Density, 352 Crystals
R2 = 0.99
1
1.2
1.4
1.6
1.8
2
2.2
2.4
1 1.5 2
X-ray Density (g/cc)
Cal
cd D
ensi
ty (
g/c
c)
Crystal structure prediction…Crystal structure prediction… successes and problems…successes and problems…
X-ray model
In 80%smallest LE ->exptl structure
Examples..(CH3)2NNO2 *TNAZ *RDX *-CL20 *FOX7 # 2, +0.4
In 20%exptl structurein top 6 lowestLE solutions
MOLPAK+
WMIN
29 coordination geometries +193 = 6,859/geometry ~ 200,000 hypothetical crystal structures
29*400-900/geom -> latticerefinement & LE calcns
# E Space # 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 X-ray 8.852 Å 12.556 13.386 106.82o
Predicted (%) 1.55 -0.53 0.98 -0.65
obsd = 2.043 g/cc
Structure prediction example, Structure prediction example, -CL20 from B3lyp/631g* model-CL20 from B3lyp/631g* model
-(NO2)8
= 2.008 g/cc
det P = 32.72 gPa
det V = 8.25 km/s
Isp = 229.7 s
-(NO2)8
= 2.105 g/cc
det P = 55.88 gPa
det V = 9.75 km/s
Isp = 266.7 s
-(NO2)12
= 1.981 g/cc, exptl = 1.978
det P = 48.74 gPa
det V = 9.35 km/s
Isp = 267.5 s
= 1.856 g/cc
det P = 32.91 gPa
det V = 8.76 km/s
Isp = 260.7 s
-(NO2)8
= 1.818 g/cc
det P = 35.21 gPa
det V = 9.08 km/s
Isp = 272.1 s
C C NO2O2N
= 1.936 g/cc
det P = 29.56 gPa
det V = 8.00 km/s
Isp = 232.3 s
C C
NO2
O2N NO2
O2N
Structure prediction -> new molecule evaluation…Structure prediction -> new molecule evaluation…
ROTPAKROTPAK……pack & adjust conformation, minimize Etotal…
Criterion: Etotal = Einter + Eintra ~ Ethresh
orientedmolecule at origin
orientedmoving molecule
move togetheruntil
Etotal ~ Ethresh
alter conformationcalc new Etotal
> Ethresh< Ethresh
Ethresh = 0.5
kcal/molfor line
no further improvement in Etotal
new orientation
closest approach
# 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
ROTPAK examples…ROTPAK examples…
O2N NO2
NO2
CUGCOW DUYREU
1
2
3 NCN
NO212
KASBAH
(NO2)7
Model (G03), ROTPAK/WMIN and X-ray torsion angle comparison…
X-ray
B3lyp/631g*model
1
2 3 NH
CH3
O
CH3
O2N
NO2
ROTPAK example…ROTPAK example…
24.6
47.1
0.0o
B3…
22.9
51.9
41.4o
X-ray
3
2
1
29.0
48.6
45.0o
RPK/ WMIN
X-ray G03 + ROTPAK + WMIN
-6.31.53
2.9119.9o
1.819.67c
1.44.85b
6.8%12.57 Åa
1.87 Å1.93 Åa
b
c
X-raymodel
G03B3lyp/631g*model
ROTPAK + WMIN
bend ~20o + N-NO2 twist
ROTPAK example…RDX bond bending…ROTPAK example…RDX bond bending…
RDX X-ray and ROTPAK-modified overlays…RDX X-ray and ROTPAK-modified overlays…
-1.31.806 (g/cc)
-0.610.709c
0.711.574b
1.213.182a (Å)
ROTPAK-
X-ray (%)
X-raydimension
RDX space group: Pbca
unit cellcomparison
Some ROTPAK challenges…Some ROTPAK challenges…
O2NC(NO2)3
O2N
O2N
NO2
N3N3
N3
N3
O
N3N3
N3
NO2
O
O
CH2-N=N=N
CH2-N=N=N
CH2-N=N=N
MOLPAK, the old and the new…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
Determine length of an axis ->make line of C1 or Ci images; assume line is a/c or b axis
For each grid, fill unit cell;calculate lattice energy
Lowest LE ->correct structure
Criteria: van der Waals radii repulsion energy total energy
Pre-LE calcn criteria:van der Waals radiicrystal density range
C1/Ci image at origin & each grid point in succession
order surviving unit cells on basis of LE
MOLPAK-2 flowchart for P2MOLPAK-2 flowchart for P211/c…/c…
Symmetry elements: inversion center,2-fold screw axis, glide plane
Make rectangular boxaround origin,0.5 Å spacing
MOLPAK-2 examples…MOLPAK-2 examples…
MOLPAK-1 failed ->no axial repeatmolecules in coodnsphere
NO2
P-1 P21
1
23
7
5
5(NO2)
P21/c
N
O
CH3
NO2
P212121
NH
HNO O
C2/c
Continuing and future work and goals…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 ‘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
•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
thanks
ARL, Picatinny Indian HeadB. ChapmanK. BaumH. ShechterP. EatonJ. Bottaro