Long Time Scale Dynamics of Reactive Materials

1. Performance Measures x.x, x.x, and x.x Long Time Scale Dynamics of Reactive Materials Computing Grand Challenge Symposium February 20th, 2008 Nir Goldman Chemistry, Materials, Earth, and Life Sciences DirectorateLawrence Livermore National Laboratory Co-PIs: Will Kuo, Evan Reed, Larry Fried, Chris Mundy (PNNL), Alessandro Curioni (IBM) Prof. Mike Klein (University of Pennsylvania)

2. H2O:CH3OH:NH3 :CO:CO2 2 : 1 : 1 : 1 : 1 We propose shock simulations of reactive materials using state of the art codes • Problem: Simulations of shock compression are necessary to: • Elucidate high P-T experiments • Study kinetics and metastable states along shock paths • Solution: New shock simulation technique (MSST) allows for orders of magnitude speed up in simulations • Shock quantum simulations are tractable for the first time! Grand Challenge: Shock simulations of interstellar ices will address questions about the origins of pre-biotic organic compounds The formation of peptide bonds is one of the greatest riddles of the origin of life

3. Shock simulations will elucidate thermodynamic path-dependent chemistry vs. Static Shock simulations • Simulation of a single thermodynamic state point • Chemistry is dependent on thermodynamic path • Reactions that occur will depend on shock velocity, initial conditions • Numerous thermodynamic states will be visited by a single shock • Equilibrate system at a given pressure, temperature • Follow with “production” run to gather data

4. polar vs. nonpolar water • We have discovered that superionic water may occur within large planets • Goncharov, Goldman, et al., PRL, 94, 125508 (2005) • Goldman et al., PRL,94, 217801 (2005) Reaction pathways generated by shocks could cause the formation of exotic states of matter 1. Exotic phases can exist at high temperature and pressure 2. Shock produce metastable states of matter • Nonpolar aqueous environments may occur near thermal vents, deep within the oceans • Shock water at these conditions could trigger pre-biotic synthesis • Bassez, J. Phys: Cond. Matt., 15, L353 (2003). Current state of the art has been limited to very small “static” simulations

5. New shock technique can revolutionize MD simulations (MSST) • The Multi-scale Shock Technique (MSST) applies a uniaxial strain on the simulation cell • simulates the thermodynamic states occurring in the shock • This allows for orders of magnitude longer simulation times We have conducted simulations of several systems: Shock compressed H2O: Hugoniot and chemical speciation Astrophysical ices: formation of C-N bonds MSSM shock dynamics of 25,688 Lennard-Jones atoms Simulations agree well with hydrodynamic steady waves

6. LLNL’s ATLAS: • 9216 AMD Opteron processors (2.4 GHz) • Peak performance: 44.2 TFlops/s • Currently #29 on Top500 list Computational and software details • Software packages: Car-Parrinello Molecular Dynamics (CPMD) and CP2K • We treat the electrons quantum mechanically at the highest level of theory possible (Density Functional Theory) • All simulations were performed with with the electrons in their ground state (Born-Oppenheimer mechanics) with large basis sets • Our simulations are extremely computationally expensive and can only be conducted using LLNL’s terascale computing facilities. • ~500,000 CPU hours needed = 1-2 months for a single astrophysical ice mixture

7. Challenge: What are the electrical properties of water inside of large planets like Neptune and Uranus? • Large magnetic fields could be regenerated due to unique properties of water under the extreme conditions of the planetary interior • Lots of experimental data exists show plateau in ionic conductivity of H2O under shock compression • Difficulty exists in describing chemistry at these conditions • For the first time we can do a direct simulation of shock compression in water Predicted H2O phase diagram Cavazzoni et al., Science, 283, 44 (1999). Questions: • How well do our simulations compare to experiments? • Can we predict the plateau in ionic conductivity? • How can we describe the phases we observe?

8. H2O at 9 km/s: = H2O = H3O+ and OH- = H+ and O2- = other We have conducted Born-Oppenheimer MD simulations of shock compressed water • Simulation details (CPMD): • 64 molecules • 120 Ry basis set cutoff • 5-10 ps • 500,000 CPU Hours Simulations of shocked water show progression to ensemble of short-lived transition states

9. Our shock Hugoniot results compare extremely well to shock compression experiments Our simulations likely intersect the Neptune isentrope at ~ 54 GPa Our result for the final density and pressure EOS match experiments very closely

10. We are able to reproduce the plateau in the ionic conductivity = Our results = Hamann and Linton, Trans. Faraday Soc. (1966). = Mitchell and Nellis, J. Chem. Phys. (1982). The plateau implies that water is fully ionized at higher pressure and temperature

11. We are able to describe the chemical mechanism for the observed ionic conduction for the first time Non-molecular H2O lifetime ≈ ion lifetime Bond cutoff choice (rc = 1.2 Å) equals the minimum in reactive flux At Vs ≥ 9 km/s, we observe a loss of molecular states and the creation of an ensemble of transition states

12. 11 km/s simulation exhibits ‘metallic’ configurations Vs (km/s) P (GPa) T(K) Egap (eV) * = averaged over low band gap configs. Band Gap closure is due to the formation of extremely short-lived O and OH negative ions

13. Water: Conclusions • Shock compression shows excellent agreement with experimental shock Hugoniot results • We observe a progression from molecular H2O, to ‘soup’ of ionic transition state • The lack of molecular species explains the plateau in ionic conductivity observed by Chau et al. • Very short-lived ionic clusters could contribute electronic states in region of band gap, causing transient metallization

14. NH3 + CO H2N-COO + H+ Challenge: Can shock compression induce abiotic production of peptides? • Experiments focus on the formation of specific amino acids: • C. Huber et al., Science, 281, 670 (1998). • P. Ehrenfreund et al., Rep. Prog. Phys., 65, 1427 (2002). • Experiments also predict only a “glancing blow” from a comet can produce peptides: Ehrenfreind et al., Astronomy and Astrophysics (1999), Blank et al., Origins of Life and Evolution of the Biosphere (2001). • High P-T will cause organics to burn up => direct hit from comet is impossible • Need range of 5-21 GPa and 412-870 K:comet traveling at 11.2 km/s, and with an angle of incidence of 7.7 - 26.5 degrees Preliminary quantum simulations show peptide bond formation at ca. 40% compression We wish to focus specifically on the formation of C-N bonds under shock compression

15. Simulations of astrophysical ice mixture under shock compression shows the formation of “pre-biotic” molecules Ice mixtures under shock compression at 4 km/s show the formation of stable peptide bonds

16. Our simulations have spanned a wide range of pressure and temperature We have investigated C-N bond formation at higher P-T than predicted by experiment

17. Simplest amino acid: H2N-COOH 670 K, 10 GPa 2. C-C linkages have formed; “pre-glycine”: H2N-CHOH-COOH 3000 K, 43.5 GPa 3. A wide variety of complex short-lived species are seen at high P-T: HN-CHOHO-CHO-CO 5000 K, 68 GPa Extremely short-lived peptide bonded molecules exist at much higher pressure and temperature than previously thought

18. Future work • Molecular Analysis: calculate C–N bond kinetics and species concentrations and lifetimes • Thermodynamics: “Rare events” simulation of different shock conditions, and free energy barrier for bond dissociation • Compositional effects: e.g., varying the starting ice composition to explore the effects of “reducing” vs. “oxidizing” conditions

19. Conclusions • We are developing a simulation capability that is uniquely possible due to LLNL’s Tera-scale computing facilities: • We observe that conductivity within Neptune and/or Uranus is enhanced by the complete ionization of water • Our studies allow for a more fundamental approach to “Origin of Life” problems. • C –– N (peptide) bond formation is activated by shock compression • Higher shock (> 8 km/s) velocities create short-lived “pre-biotic” species that can be quenched out upon cooling “Perhaps it is time to stop this seemingly endless series of phenomenological experiments and to concentrate instead on quantifying reaction rates, the relative stabilities of synthesized compounds, and the effects of variables such as pressure and temperature.” - Everett L. Shock, Wash U. in St. Louis., Nature, 2002.

20. Other “thermostatting” techniques do not compare well to NEMD NEMD equivalent MSST Constant Pressure Hugoniostat equivalent Constant Volume Hugoniostat equivalent In EOS modeling of high explosives, Hugoniotstats are inaccurate for intermediate kinetic states Reed et al., PRE, 2006. MSST is the only shock compression MD technique to accurately reproduce kinetics