QTPIE: A new charge model for arbitrary geometries and systems

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Slides for my talk at the 234th ACS National Meeting (Fall 2007) in Boston, MA.

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QTPIE: A new charge model for arbitrary geometries and systems

  1. 1. QTPIE: A new charge model for arbitrary geometries and systems Jiahao Chen and Todd J. Martínez Department of Chemistry and the Beckman Institute Poster: 7:30-9:30 tonight, BCEC Exhibit Hall B2, #107
  2. 2. Polarization effects are important in classical molecular dynamics <ul><li>Structure of water improved when polarization is accounted for, even if implicitly 1 </li></ul><ul><li>Needed to describe local environmental effects, e.g. hydration of chloride in water clusters 2 </li></ul>1 Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. J. Phys. Chem. 91 , 1987 , 6269-71. 2 Stuart, S. J.; Berne, B. J. J. Phys. Chem. 100 , 1996 , 11934 -11943. OPLS/AA Non-polarizable force field TIP4P/FQ Polarizable force field
  3. 3. <ul><li>Polarizable point dipole models </li></ul>How to represent explicit polarization in classical MD? Review: Yu, H.; van Gunsteren, W. F.; Comput. Phys. Commun. 172 (2005), 69-85. +q ,   -q ,   Induced dipoles calculated from site polarizabilities  fixed calculated
  4. 4. How to represent explicit polarization in classical MD? <ul><li>Polarizable point dipole models </li></ul><ul><li>Drude oscillator/charge-on-spring/shell models </li></ul>Review: Yu, H.; van Gunsteren, W. F.; Comput. Phys. Commun. 172 (2005), 69-85. spring k charge -Q >> q mass m << M charge q+Q mass M-m calculated
  5. 5. <ul><li>Polarizable point dipole models </li></ul><ul><li>Drude oscillator/charge-on-spring/shell models </li></ul><ul><li>Electronegativity equalization/charge equilibration/fluctuating-charge models </li></ul><ul><ul><li>Model polarization as a type of charge transfer </li></ul></ul>How to represent explicit polarization in classical MD? Review: Yu, H.; van Gunsteren, W. F.; Comput. Phys. Commun. 172 (2005), 69-85. calculated
  6. 6. Fluctuating-charge models map molecules onto electrical circuits screened Coulomb interaction chemical hardness electro- negativity molecule More electropositive More electronegative 0 V         - Voltage + electric potential (inverse) capacitance electrical circuits Coulomb interaction
  7. 7. QEq, a typical fluctuating-charge model <ul><li>Energy minimized with respect to charges subject to constraint on total charge Q </li></ul><ul><li>Screened Coulomb interactions </li></ul><ul><li>s -type Slater orbitals </li></ul>Rappé, A. K.; Goddard, W. A. , J. Phys. Chem. 95 (1991), 3358-3363 .
  8. 8. Limitations of QEq <ul><li>No out-of-plane dipole polarizability </li></ul><ul><li>Overestimates in-plane dipole polarizability </li></ul><ul><li>Unphysical charge distributions predicted for non-equilibrium geometries </li></ul><ul><li>Cause: no distance penalty for charge transfer </li></ul>voltage distance        
  9. 9. QTPIE, our new charge model <ul><li>Charge-transfer with polarization current equilibration </li></ul><ul><li>Voltage attenuates with increasing distance </li></ul>J. Chen and T. J. Martínez , Chem. Phys. Lett. 438 (2007) 315 -3 20. voltage distance        
  10. 10. Features of QTPIE <ul><li>Correct dissociation limit for uncharged fragments </li></ul><ul><li>Minimally parameterized in terms of chemically meaningful quantities (electronegativites and hardnesses) </li></ul><ul><li>Can obtain results for electrostatic properties comparable to those from more sophisticated force fields </li></ul>
  11. 11. Dissocation of H 2 O in QEq and QTPIE <ul><li>Correct asymptotics </li></ul><ul><li>Charge separation on OH fragment retained </li></ul>equilibrium geometry R QEq QTPIE ab initio QTPIE prediction improved over QEq without reoptimizing parameters ab initio = DMA charges from CASSCF(6/4)/STO-3G wavefunction
  12. 12. Cooperative polarization in water <ul><li>Dipole moment of water increases from 1.854 Debye 1 in gas phase to 2.95±0.20 Debye 2 at r.t.p. liquid phase </li></ul><ul><li>Polarization enhances dipole moments </li></ul><ul><li>Water models with implicit or no polarization can’t describe local electrical fluctuations </li></ul>1 D. R. Lide, CRC Handbook of Chemistry and Physics , 73rd ed., 1992 . 2 A. V. Gubskaya and P. G. Kusalik, J. Chem. Phys. 117 (2002) 5290-5302. +
  13. 13. Creating a water model with QTPIE <ul><li>Replace implicit polarization in TIP3P 1 by explicitly polarizable charges using QTPIE and QEq </li></ul><ul><li>QTPIE, QEq implemented in TINKER </li></ul><ul><li>Reparameterized to reproduce ab initio dipole moments and anisotropic polarizabilities of a single water molecule </li></ul><ul><ul><li>ab initio = DF-LMP2/aug-cc-pVDZ </li></ul></ul>1 Jorgensen, W. L.; et al. , J. Chem. Phys. 79 (1983) 926-935.
  14. 14. New parameters for TIP3P/QTPIE and TIP3P/QEq <ul><li>Mulliken electronegativities and </li></ul><ul><li>Parr-Pearson hardnesses </li></ul>1 Rappé, A. K.; Goddard, W. A. , J. Phys. Chem. 95 (1991), 3358-3363. 2 Calculated from ionization potentials and electron affinities in NIST Webbook. 12.157 20.680 20.680 13.364   12.844 10.125 10.125 13.890   7.540 8.125 8.285 8.741   7.176 5.116 4.960 4.528   Expt. 2 QEq QTPIE Original 1 eV
  15. 15. Dipole response of linear water chains <ul><li>Use parameters from single water molecule to model chains of water molecules </li></ul><ul><li>Compared with: </li></ul><ul><ul><li>Gas phase experimental data 1 </li></ul></ul><ul><ul><li>Ab initio DF-LMP2/aug-cc-pVDZ </li></ul></ul><ul><ul><li>AMOEBA 2 , a point polarizable dipole force field </li></ul></ul>1 Murphy, W. F. J. Chem. Phys. 67 , 1977 , 5877-5882. 2 Ren, P.; Ponder, J. W. J. Phys. Chem. B 107 , 2003 , 5933-5947.. planar (0° twist) twisted (90°)
  16. 16. Mean dipole moment per water planar
  17. 17. Mean dipole moment per water twisted
  18. 18. TIP3P/QTPIE predicts dipoles well <ul><li>Simpler, yet comparable to AMOEBA </li></ul>planar twisted 4 4 3 14 No. of nonzero electrostatics parameters TIP3P/QTPIE TIP3P/QEq TIP3P AMOEBA Water model
  19. 19. Conclusions <ul><li>Distance-dependent electronegativity difference leads to correct asympotic behavior of dissociating neutral fragments </li></ul><ul><li>New TIP3P/QTPIE water model predicts dipole moments better than TIP3P/QEq </li></ul><ul><li>TIP3P/QTPIE models polarization effects with results comparable to more expensive force fields </li></ul>
  20. 20. Acknowledgments <ul><li>Prof. Todd J. Martínez </li></ul><ul><li>Martínez Group </li></ul><ul><li>Funding from DOE DE-FG02-05ER46260 </li></ul><ul><li>Poster </li></ul><ul><li>Tonight 7:30-9:30 </li></ul><ul><li>BCEC Exhibit Hall B2 </li></ul><ul><li>#107 </li></ul>
  21. 21. Out-of-plane polarizability per water planar
  22. 22. Out-of-plane polarizability per water twisted
  23. 23. In-plane polarizability per water planar
  24. 24. In-plane polarizability per water twisted
  25. 25. Dipole-axis polarizability per water planar
  26. 26. Dipole-axis polarizability per water twisted
  27. 27. TIP3P/QTPIE doesn’t predict polarizabilities well <ul><li>Identical to TIP3P/QEq </li></ul><ul><li>No out of plane polarizability </li></ul><ul><li>In-plane components underestimated </li></ul>twisted planar out of plane in plane dipole axis

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