ReaxFF for Vanadium
and Bismuth Oxides
Kim Chenoweth
Force Field Sub-Group Meeting
January 20, 2004
Overview
•
•
•
•
•
Significance of a Bi/V force field
ReaxFF: general principles
Force field optimization for V
Force field optimization for Bi
Future work
Designing a Better Catalyst - I
• 85% of industrial organic chemicals are currently produced by catalytic
processes
• 25% are produced by heterogeneous oxidation catalysis such as
ammoxidation
CH2=CHCH3 + NH3 + 3/2 O2
Cat
CH2=CHCN + 3 H2O
• Bi-molybdates are currently used as the catalyst
• Use of alkanes as a cheaper feedstock requires design of a selective
catalyst
• Promising catalysts are complex oxides containing Mo, V, Te, X, and O
where X is at least one other element
 Bismuth is one of the 19 elements listed in the Mitsubishi patent
Designing a Better Catalyst - II
• Low-MW alkenes (i.e. ethene and propene) can be formed via nonoxidative dehydrogenation (ODH) of the corresponding alkane
• Supported vanadia is the most active and selective simple metal oxide for
alkane ODH1
 Due to its reducible nature, it leads to rapid redox cycles necessary for catalytic
turnover
 Local structure strongly influences ODH reaction rates and selectivity
• Force field would allow for the study of large and complex systems with
many atoms
 Generate interesting structures for further study using QC methods
 Optimize ratio of the various metals in the catalyst
 Elucidate the purpose of the different metals
1Argyle
et al, J. Catal. 2002, 208, 139
ReaxFF
years
Bridging the gap between QC and EFF
Atoms
Molecular
conformations
Electrons
Bond formation
Design
Time
FEA
MESO
MD
ReaxFF
10-15
QC
Grids
Grains
Empirical methods:
• Study large system
• Rigid connectivity
QC Methods:
• Allow reactions
• Expensive
ReaxFF:
• Simulate bond
formation in larger
molecular systems
Empirical
force fields
ab initio,
DFT,
HF
Ångstrom
Kilometers
Distance
ReaxFF: Energy of the System
Esystem  Ebond  EvdW aals  ECoulomb  Eval  Etors
2-body
3-body
4-body
 Eover  Eunder
multi-body
• Similar to empirical non-reactive force fields
• Divides the system energy into various partial energy contributions
Important Features in ReaxFF
• A bond length/bond order relationship is used to obtain smooth transition
from non-bonded to single, double, and triple bonded systems.
 Bond orders are updated every iteration
• Non-bonded interactions (van der Waals, coulomb)
 Calculated between every atom pair
 Excessive close-range non-bonded interactions are avoided by shielding
• All connectivity-dependent interactions (i.e. valence and torsion angles) are
made bond-order dependent
 Ensures that their energy contributions disappear upon bond dissociation
• ReaxFF uses a geometry-dependent charge calculation scheme that
accounts for polarization effects
ReaxFF as a Transferable Potential
General Rules:
 No discontinuities in energy or forces even during
reactions
 No pre-defined reactive sites or reaction pathways
 Should be able to automatically handle coordination
changes associated with reactions
 One force field atom type per element
 Should be able to determine equilibrium bond lengths,
valence angles, etc from chemical environment
Strategy for Parameterization of ReaxFF
1.
2.
3.
4.
5.
Identify important interactions to be optimized for relevant
systems
Build QC-training set for bond dissociation and angle bending
cases for small clusters
Build QC-training set for condensed phases to obtain equation
of state
Force field optimization using
1. Metal training set
2. Metal oxide clusters and condensed phases
Applications
Vanadium Training Set
1st row transition metal (4s23d3)
• Cluster
 Bonds
 -Normal, under-, and overcoordinated systems
 Angles
 O-V=O, V-O-V, O=V=O
• Successive bond
dissociation of
oxygen in V4O10
• Condensed Phase
 Metal
 BCC, A15, FCC, SC,
Diamond
 Metal Oxide
 VO (II)
• FCC
 V2O3 (III)
• Corundum
 VO2 (IV)
• Distorted rutile
 V2O5 (V)
• Layered octahedral
Bulk Metal - Vanadium
ReaxFF
90
90
80
80
70
70
E/atom (kcal/mol)
E/atom (kcal/mol)
QC
60
50
40
30
60
50
40
30
20
20
10
10
0
0
5
10
15
20
Vol./atom (Å^3)
25
30
Diamond
SC
FCC
A15
BCC
5
10
15
20
25
Vol/atom (Å^3)
•ReaxFF reproduces EOS and properly predicts instability of lowcoordination phases (SC, Diamond)
30
Bond
Dissociation
in
VO2OH
V=O Bond Dissociation
V-O Bond Dissociation
190
150
170
Relative Energy (kcal/mol)
170
Relative Energy (kcal/mol)
190
QM (singlet)
QM (triplet)
ReaxFF
130
110
90
70
50
30
150
130
110
90
70
50
30
10
10
-10
-10
0.5
1.5
2.5
3.5
Bond Distance (Å)
4.5
QM (singlet)
QM (triplet)
ReaxFF
0.5
1.5
2.5
3.5
Bond Distance (Å)
4.5
V=O Bond Dissociation in V4O10
V=O Bond Dissociation
190
Relative Energy (kcal/mol)
170
150
130
110
90
70
50
30
10
-10
0.5
QM (singlet)
QM (quintet)
ReaxFF
1.5
2.5
3.5
Bond Distance (Å)
4.5
Angle Distortion in V2O5
V-O-V Angle
O=V-O Angle
V-O-V Angle
120
ReaxFF
QC
15
Relative Energy (kcal/mol)
Relative Energy (kcal/mol)
20
O-V=O Angle
10
5
0
-5
ReaxFF
QC
100
80
60
40
20
0
-20
75
100
125
150
Angle (Degrees)
175
200
50
75
100
125
Angle (Degrees)
150
175
Angle Distortion in VO2
O=V=O Angle
O=V=O Angle
Relative Energy (kcal/mol)
120
ReaxFF
QC
100
80
60
40
20
0
-20
50
75
100
125
Angle (Degrees)
150
175
Angle Distortion in V2O6
V-O-O Angle
V-O-O Angle
Relative Energy (kcal/mol)
30
ReaxFF
QC
25
20
15
10
5
0
-5
50
75
100
125
Angle (Degrees)
150
175
Charge Analysis for VxOy Clusters in Training Set
1.2
1.2
4
0.8
1
Mullikan Charges
2
3
0.2
-0.3
1
2
0.6
0.4
0.2
0
-0.2
-0.8
-0.4
-1.3
-0.6
1
Mullikan Charges
Mullikan Charges
0.7
3
1
2
3
Atom Number
4
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
3
4
2
2
Atom Number
ReaxFF
QC
6
1
3
5
7
1
2
3
4
Atom Number
5
6
7
Mullikan Charges (D
0.8
Charge Analysis for VxOY Clusters0.4in Literature
0
(QC data taken from Calatayud et al, J. Phys. Chem. A 2001,
105, 9760.)
0.8
0.4
0
-0.4
-0.8
-0.8
0.8
2
3
Atom Number
4
1.2
1 2 0.8
3 4 5 6 7 8
0.4
Atom
Number
0.4
0
-0.4
-0.8
1
0
-0.4
-0.8
1
2
3
4
Atom Number
1.2
5
1
2
3
4
Atom Number
1.2
Mullikan Charges
Mullikan Charges
-0.4
Mullikan Charges
1.2
Mullikan Charges
Mullikan Charges
1.2
0.8
0.4
0
-0.4
-0.8
0.8
0.4
0
-0.4
-0.8
1
2
3
4
5
Atom Number
6
1
2
3
4
5
Atom Number
6
5
7
9
11
Atom Number
13
Mullikan Charges
Mullikan Charges
1.2
0.8
0.4
0
-0.4
-0.8
1.2
0.8
0.4
0
-0.4
-0.8
1
2
3
4 5 6 7
Atom Number
8
9
ReaxFF
QC
1
3
5
Bismuth Training Set
Common oxidation states: 3, 5
• Cluster
 Bonds
 -Normal, under-, and overcoordinated systems
 Angles
 Bi-Bi=O, O=Bi-O
• Condensed Phase
 Metal
 HCP, SC, BCC, A15, FCC,
Diamond
 Metal Oxide
 BiO (II)
• Trigonal
 a-Bi2O3 (III)
• Monoclinic
 b-Bi2O3 (III)
• Distorted cubic
 Bi2O4 (BiIIIBiVO4)
• Monoclinic
 BiO2 (IV)
• Cubic
Bulk Metal - Bismuth
QC
ReaxFF
10
10
E/atom (kcal/mol)
60
70
60
0
50
-5
40
20
30
40
50
Vol./atom (Å^3)
30
E/atom (kcal/mol)
70
80
5
20
10
E/atom (kcal/mol)
E/atom (kcal/mol)
80
5
Diamo
nd
SC
0
FCC
A15
50
-5
40
20
30
BCC
40
50
Vol./atom (Å^3)
30
20
10
0
Diamond
FCC
BCC
-10
SC
A15
HCP
0
-10
10
20
30 40 50 60
Vol./atom (Å^3)
70
80
10
20
30 40 50 60
Vol./atom (Å^3)
70
80
Relative Stabilities of V and Bi Bulk Phases
Bismuth
Vanadium
Relative Energies (kcal/mol)
ReaxFF
QM
BCC
0.00
0.00
A15
-2.51
-2.00
FCC
-6.34
-6.56
SC
-27.43
-24.18
Diamond
-71.05
-63.19
Vanadium
Bismuth
Relative Energies (kcal/mol)
ReaxFF
QM
HCP
0.00
0.00
SC
-0.36
-0.42
BCC
-0.60
-0.61
A15
-1.53
-2.94
Diamond
-6.12
-4.52
Cohesive Energies (kcal/mol)
ReaxFF
Lit.
123.3
122.5
50.4
50.3
Application: Melting Point of Vanadium
55 molecules
900 K
1700 K
2500 K
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
1900 K
• Melting point of Vanadium = 2163 K
• Melting point obtained from simulation ~ 1900 K
1700 K
900 K
Application: Melting Point of Vanadium
147 molecules
900 K
1700 K
2500 K
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
2000 K
• Melting point of Vanadium = 2163 K
• Melting point obtained from simulation ~ 2000 K
1700 K
900 K
Future Work
• Bismuth oxide force field training set:
 Optimization of Bi oxide force field
 Add bond dissociation and bond angles for clusters
 Add bismuth oxide condensed phases
Add to training set and
continue optimizing force field
• Vanadium oxide force field training set:
 Further optimization of vanadium oxide force field
 Add successive V=O bond dissociation for V4O10
 Add vanadium oxide condensed phases
Add to training set and
continue optimizing force field