Poster presented at the Fall 2007 ACS national meeting.

1. QTPIE: A minimal extension of Goddard’s QEq model with correct dissociation
Jiahao Chen and Todd J. Martínez
Department of Chemistry, Frederick Seitz Materials Research Laboratory, and the Beckman Institute
University of Illinois at Urbana-Champaign
5
Dipole response of linear water chains
QTPIE , our new charge model
Polarization effects are important in classical
• Charge-transfer with polarization current equilibration
molecular dynamics simulations • Use parameters from single water molecule to model chains of waters
• Voltage attenuates with increasing distance • Compared with gas phase experimental data10, ab initio (DF-LMP2/aug-
• Structure of water improved when polarization is accounted for,
cc-pVDZ), and AMOEBA11, a point polarizable dipole force field
even if implicitly1 voltage
• Needed to describe local environmental effects, e.g. hydration of
Mean dipole moment per water
chloride in water clusters2 η2
2.6
Planar
( /N)/Debye
η2
OPLS/AA
OPLS/FQ
Non-polarizable force field
Polarizable force field 2.5
distance AMOEBA
DF-LMP2/aug-cc-pVDZ
Features of QTPIE 2.4
TIP3P/QTPIE
3
How to represent explicit polarization? TIP3P
• Correct dissociation limit for uncharged fragments TIP3P/QEq
2.3
• Minimally parameterized in terms of chemically meaningful quantities
• Polarizable point dipole models
(electronegativites and hardnesses) 2.2
• Can obtain results for electrostatic properties comparable to those from 2.1
more sophisticated force fields
+q, α1 -q, α2 2.0
Dissocation of water in QEq and QTPIE
Induced dipoles calculated from site polarizabilities • Correct asymptotics 1.9
gas phase (experimental)
• Drude oscillator/charge-on-spring/shell models • Charge separation on OH fragment retained Number of water molecules, N
1.8
0 5 10 15 20 25 30 35 40
1.0
q/e
Twisted
2.6
( /N)/Debye
2.5
0.5 AMOEBA
DF-LMP2/aug-cc-pVDZ
charge -Q >> q
charge q+Q
mass m << M R/Å
mass M-m 2.4
TIP3P/QTPIE
TIP3P
spring k 0.0
TIP3P/QEq
• Electronegativity equalization/charge equilibration/fluctuating- 2.3
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
charge models QTPIE
2.2
ab initio
-0.5
QEq 2.1
2.0
-1.0
Model polarization as a type of charge transfer QTPIE prediction improved over QEq without reoptimizing parameters
1.9
ab initio = DMA charges from CASSCF(6/4)/STO-3G wavefunction gas phase (experimental)
Number of water molecules, N
Cooperative polarization in water 1.8
0 5 10 15 20 25 30 35 40
chemical
electro-
atoms in
TIP3P/QTPIE predicts dipoles well
• Dipole moment of water increases from 1.854 Debye6 in gas phase to
hardness screened
negativity
molecule
Coulomb 2.95±0.20 Debye7 at r.t.p. liquid phase
(inverse)
electric
electrical • Simpler, computationally cheaper, yet results comparable to AMOEBA
interaction
capacitance
potential • Polarization enhances dipole moment
circuits
• Water models with implicit or no polarization can’t describe local electri- Water model AMOEBA TIP3P TIP3P/QEq TIP3P/QTPIE
η1
No. of electrostatics parameters
14 3 4 4
cal fluctuations
χ2
η2
Conclusions
0V
• Distance-dependent electronegativity difference leads to correct asympot-
4
QEq , a typical fluctuating-charge model ic behavior of dissociating neutral fragments
• Energy minimized with respect to charges subject to constraint on total • New TIP3P/QTPIE water model predicts dipole moments better than
Creating a water model with QTPIE
charge Q TIP3P/QEq
• Replace implicit polarization in TIP3P8 by explicitly polarizable charges
• TIP3P/QTPIE models polarization effects with results comparable to
using QTPIE and QEq
more expensive force fields
• QTPIE, QEq implemented in TINKER
• Screened Coulomb interactions
Acknowledgments
• Reparameterized to reproduce ab initio dipole moments and anisotropic
polarizabilities of a single water molecule Prof. Todd J. Martínez
• ab initio = DF-LMP2/aug-cc-pVDZ Martínez Group
• s-type Slater orbitals New parameters for Funding from DOE DE-FG02-05ER46260
References
TIP3P/QTPIE and TIP3P/QEq
Limitations of QEq 1. Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. J. Phys. Chem. 91
• Mulliken electronegativities and Parr-Pearson hardnesses
(1987) 6269-71.
• No out-of-plane dipole polarizability
2. Stuart, S. J.; Berne, B. J. J. Phys. Chem. 100 (1996) 11934 -11943.
• Overestimates in-plane dipole polarizability
3. Yu, H.; van Gunsteren, W. F. Comput. Phys. Commun. 172 (2005) 69-
• Unphysical charge distributions predicted for non-equilibrium geometries
85.
• Cause: no distance penalty for charge transfer
4. Rappé, A. K.; Goddard, W. A. J. Phys. Chem. 95 (1991) 3358-3363.
Original4 Expt.9
eV QTPIE QEq
5. Chen, J.; Martínez, T. J. Chem. Phys. Lett. 438 (2007) 315-320.
4.528 4.960 5.116 7.176
χH
6. Lide, D. R. CRC Handbook of Chemistry and Physics, 73rd ed., 1992.
η2 η2
8.741 8.285 8.125 7.540
χO
7. Gubskaya, A. V.;Kusalik, P. G. J. Chem. Phys. 117 (2002) 5290-5302.
13.890 10.125 10.125 12.844
ηH 8. Jorgensen, W. L.; et al., J. Chem. Phys. 79 (1983) 926-935.
13.364 20.680 20.680 12.157
ηO
distance
9. NIST Webbook.
10. Murphy, W. F. J. Chem. Phys. 67 (1977) 5877-5882.
11. Ren, P.; Ponder, J. W. J. Phys. Chem. B 107 (2003) 5933-5947.