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Polarization and charge transfer in classical molecular dynamics
1. Polarization and charge
transfer in classical
molecular dynamics
Jiahao Chen
Martínez Group
Chemistry, MRL and Beckman, UIUC
2. Methods of computational chemistry
ˆ
HΨ = EΨ What is the charge distribution?
direct density coarse-
ab initio semiempirical molecular continuum
numerical functional grained
theories methods models (MM) electrostatics
quadrature theory models
more variables less variables
numerical quadrature, classical coarse-
finite element
e.g. real-time path ab initio molecular dynamics molecular grained
methods
integral propagators dynamics dynamics
ˆ ˙
HΨ = iΨ What does the system do?
4. Molecular models/force fields
Typical energy function
E= b∈bonds
kb (rb − req,b )2+
a∈angles
κa (θa − θeq,a )2 +
d∈dihedrals n
lnd cos (nπ)
bond stretch angle torsion dihedrals
+ -
12 6
qi qj σij σij
+ r
+ ij
rij
−
rij
i<j∈atoms ij i<j∈atoms
electrostatics dispersion
5. Why care about polarization
and charge transfer?
Unique to condensed
phases, where most
chemistry and biology
happens
6. Polarization in chemistry
• Effect of local environment in liquid phases
• Ex. 1: Stabilizes carbonium in lysozyme
• Ex. 2: Hydrates chloride in water clusters
TIP4P/FQ OPLS/AA
polarizable non-polarizable
force field force field
1. A Warshel and M Levitt J. Mol. Biol. 103 (1976), 227-249.
2. SJ Stuart and BJ Berne J. Phys. Chem. 100 (1996), 11934 -11943.
7. 3 models for
polarization
Review: H Yu and WF van Gunsteren Comput. Phys. Commun. 172 (2005), 69-85.
8. Drude oscillators
or charge-on-spring
or shell models
Q
R k Ideal spring
q−Q
Response = change in R
Review: H Yu and WF van Gunsteren Comput. Phys. Commun. 172 (2005), 69-85.
9. Inducible dipoles
α1 α2
µinduced,1 µinduced,2
Response = change in induced dipoles
Review: H Yu and WF van Gunsteren Comput. Phys. Commun. 172 (2005), 69-85.
10. Fluctuating charges
χ1 , η1
-0.3
charge transfer = 0.5 charge transfer = 0.2 e
-1.1 χ2 , η2
+1.4 charge transfer = 0.9 e
χ3 , η3
Response = change in atomic charges
Review: H Yu and WF van Gunsteren Comput. Phys. Commun. 172 (2005), 69-85.
11. Better Electrostatics
Polari- Charge
Model Cost
zation transfer
qi qj
r
i<j∈atoms ij
Pairwise fixed charges ❙
Drude oscillator ✓ ❙❙
Inducible dipoles ✓ ❙❙❙❙❙❙
Fluctuating charges ✓ ✓ ❙❙❙
12. QEq, a fluctuating-
charge model
E= qi χi + qi qj Jij
i atomic i<j screened
electronegativities Coulomb
“voltages” interactions
φ2 (r1 )φ2 (r2 )
i j
Jij = dr1 dr2
R3×2 |r1 − r2 |
φi (r) = Ni |r − Ri |ni −1 e−ζi |r−Ri |
AK Rappé and WA Goddard III J. Phys. Chem. 95 (1991), 3358-3363.
13. QEq has wrong asymptotics
1.0
q/e
Na Cl
R
0.8
χ1 − χ2
q=
J11 + J22 − J12
0.6
QEq
0.4 asymptote ~ 0.43 ≠ 0
0.2
ab initio
0.0 R/Å
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
14. QTPIE: our new model
E= qi χi + qi qj Jij
i i<j
replace atomic
pji χi kij S ij electronegativities with
distance-dependent pairwise
i<j electronegativities
or “potential differences”
Sij = φi (r)φj (r)dr overlap integral
R3
φi (r) = Ni |r − Ri |ni −1 e−ζi |r−Ri | J Chen and T J Martínez, Chem. Phys. Lett. 438 (2007), 315-320.
15. QTPIE has correct limit
1.0
q/e
Na Cl
R
0.8
χ1 − χ2
q=
J11 + J22 − J12
0.6
QEq
0.4 (χ1 − χ2 )S12
q=
J11 + J22 − J12
QTPIE
0.2
ab initio
0.0 R/Å
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
16. Execution times
TImes to solve the QTPIE model
4
10
N6.20
1000
N1.81
100
Solution time (s)
10
1
0.1
Bond-space SVD
Bond-space COF
Atom-space iterative solver
Atom-space direct solver
0.01
4 5
10 100 1000 10 10
N
Number of atoms
J Chen and T J Martínez, in preparation.
17. Cooperative
polarization in water
+ −→
• Dipole moment of water increases from 1.854
Debye1 in gas phase to 2.95±0.20 Debye2 at r.t.p.
(liquid phase)
• Polarization enhances dipole moments
• Missing in models with implicit or no polarization
1. D R Lide, CRC Handbook of Chemistry and Physics, 73rd ed., 1992.
2. AV Gubskaya and PG Kusalik J. Chem. Phys. 117 (2002) 5290-5302.
18. Polarization in water chains
• Use parameters from single water molecule
to model chains of waters
• Compare QEq and QTPIE with:
๏ Gas phase experimental data1
๏ Ab initio DF-LMP2/aug-cc-pVDZ ˆ
HΨ = EΨ
๏ AMOEBA2, an inducible dipole model
๏ TIP3P, a common implicit polarization model
1. WF Murphy J. Chem. Phys. 67 (1977), 5877-5882.
2. P Ren and JW Ponder J. Phys. Chem. B 107 (2003), 5933-5947.
19. Dipole moment per water
2.6
( /N)/Debye
2.5
AMOEBA
DF-LMP2/aug-cc-pVDZ
2.4
TIP3P/QTPIE
TIP3P
2.3 TIP3P/QEq
2.2
2.1
2.0
1.9
gas phase (experimental)
Number of water molecules, N
1.8
0 5 10 15 20 25 30 35 40
20. Charge transfer in 15 waters
Charges per molecule in chain of 15 water molecules
0.03
Charge on N molecule QTPIE
QEq
0.02 Mulliken/DF-LMP2/aug-cc-pVDZ
th
0.01
0
-0.01
-0.02
Molecule No. N
-0.03
1 3 5 7 9 11 13 15
21. Summary
• Polarization and charge transfer are important
effects usually neglected in classical MD
• Our new charge model corrects deficiencies in
existing fluctuating-charge model at similar
computational cost
• We obtain quantitative polarization and qualitative
charge transfer trends in linear water chains
22. Acknowledgments
Prof. Todd J. Martínez
Martínez Group and friends
$: DOE