Teaching an old water model new tricks: (pp)F3C, a simple protonizable water.

1. Teaching an old water model new tricks: (pp)F3C, a simple protonizable water.  Julius Su, Goddard group ff Subgroup presentation.

2. Proton dynamics are integral to the function of many key systems CsHSO4 solid acid Protein proton shuttle Nafion polymer HSAPO-34 zeolite

3. Protonizable molecular dynamics: “difficult” effects bond breaking and forming +1 +1/3 +1/3 multibody effects and polarization +1/3 electrostatics Essentially need a reactive ff over all solvent molecules!

4. ppF3C design philosophy -2q • Use a simple validated water model. +q +q 2. Use additional terms for the protons and protonatable sites only. extended description http://biot.alfred.edu/~lewis/BPTI_WEB_1/BPTI_0/Images/bpti_1.html 3. Use terms easily implementable in current generation force fields.

5. The polarizable proton (ppF3C) model: energy components Short range angular (bonding) F3C water model (electrostatics, VDW) Polarizable proton shell (3-body effects) simple extension of existing F3C model

6. Van der Waals: 12-6 interaction (no proton) The polarizable proton model: F3C portion proton interacts only as point charge H+ Electrostatics: pt charge qH = +0.410eqH+ = 1.000e qO = –0.820e

7. DEscreen q f The polarizable proton model: short range angular HOH normal vector q r provides bonding dependence f a = 88.7 kcal/mol b = 0.60 Å-1 a = 0.63 based on previously observed dependence

8. +q +q The polarizable proton model: polarizable shell r proton (+1) one pt. charge per protonatable site r’ shell (-1) harmonic restoring force (polarizability) Gaussian shell density (screening) q = 4.02e, re = 0.92 A k = 26274.2 kcal/mol/A2 reproduces three body effect

9. q r f PP-F3C fitting to monomer-proton geometries good fit but slightly too tightly bound at long range. S2/N = 89.1 q = 90o f = 0o f = 90o q = 0o

10. 1 2 3 rDH 4 1 5 2 3 rDA 4 5 rDH rDA PP-F3C fitting to dimer-proton geometries All angle combinations represented: 1 1 2 1 3 1 1 2 2 2 3 2 1 3 2 3 3 3 1 4 2 4 1 5 2 5 Scan over proton/water distances: S2/N = 45.2 excellent fit

11. Short range angular term: inversion barrier (r = 1.0 A, tetrahedral water) q = 0o q = 90o q = 180o PP-F3C MP2/6-31G** get almost 2/3 of the inversion barrier correct DEbarrier = 2.6 kcal/mol (PP-F3C) 4.1 kcal/mol (MP2/6-31G**)

12. H+ H+ Swapping equivalent protons pick closest oxygen, random hydrogen on it. accept swap with Histogram of DE shows most proposed swaps are “uphill” DE (kcal/mol)

13. Estimating a diffusion constant for H+ proton is quickly trapped between two waters periodic water (12 A)3, 10 ps run 298 K, Ewald sum. can adjust hopping temperature to fit D=7.8x10-5 cm2/sec

14. Reality vs. ppF3C partial bonds and charges equivalent proton swapping

15. Reality vs. ppF3C partial bonds and charges equivalent proton swapping nonisotropic/nonuniform electron density anisotropic bonding term

16. Reality vs. ppF3C partial bonds and charges equivalent proton swapping nonisotropic/nonuniform electron density anisotropic bonding term polarizable species, point proton point species, polarizable proton