Quantum chemistry traditionally solves the time-independent, zero temperature electronic Schrödinger equation, assuming separability of the electronic and nuclear degrees of freedom. This provides potential energy surfaces for use in molecular dyna-mics simulations to understand finite temperature and time-dependent effects. We take a different approach-extending quantum chemistry into the time domain, bridging the gap between traditional molecular dynamics (what are the atoms doing?) and quantum chemistry (what are the electrons doing?). We include quantum mechanical effects on the behavior of the electrons and the atoms by simultaneously solving the electronic and nuclear Schrödinger equations. This "ab initio multiple spawning" (AIMS) method opens exciting possibilities in modeling chemistry. Rearrangement of chemical bonds, tunneling, and dynamics on multiple electronic states are all treated correctly without ad hoc assumptions.
We are especially interested in electronic excited states, where the assumption of electron-nuclear separability breaks down. Using AIMS, we investigated fundamental photochemical reactions-quenching of excited metal atoms, cis-trans isomerization in ethylene and butadiene, and ring-opening of cyclobutene. In each case we found that conventional explanations required modification. This research furthers the understanding of complex molecular dynamics on multiple electronic states during photochemical reactions. Our goal is AIMS for reactions in complex environments, whether they be normal solvents (e.g., water), solid cages (e.g.,zeolites), or portions of a protein. We are developing methods to address solvent effects on photochemistry and spectroscopy, with ultimate application to biologically relevant molecules such as visual pigments.
Because of tunneling effects, proton transfer reactions require quantum treatment of the nuclei. We recently performed the first ab initio molecular dynamics simulation of real-time tunneling dynamics, simulating intramolecular proton transfer in malonaldehyde. Future directions include AIMS studies of coupled electron and proton transfer reactions, which are important in biological systems and possibly for designing molecular electronic devices.
Finally, we use novel quantum chemistry methods to elucidate the function of biologically relevant metallo-proteins-currently, the reaction mechanism of cytochrome c oxidase. This final enzyme in the respiratory cycle reduces oxygen. We investigate the nature of spin coupling between transition metal centers, the role of tyrosyl radicals in the mechanism, and the coupling of electron transfer and proton transfer in the enzyme active site.
1) "Force-induced Activation of Covalent Bonds in Mechanoresponsive Polymeric Materials," D.A. Davis, A. Hamilton, J. Yang, L.D. Cremar, D. Van Gough, S.L. Potisek, M.T. Ong, P.V. Braun, T.J. Martínez, J.S. Moore, S.R. White, and N.R. Sottos, Nature, 459, 68-72 (2009).
2)" Photodynamics in Complex Environments: Ab Initio Multiple Spawning Quantum Mechanical/Molecular Mechanical Dynamics," A.M. Virshup, C. Punwong, T.V. Pogorelov, B. Lindquist, C. Ko, and T.J. Martínez, J. Phys. Chem., Invited centennial feature article, 113B, 3280-3291 (2009).
3) "Quantum Chemistry on Graphical Processing Units. 2. Direct Self-Consistent Field Implementation," I.S. Ufimtsev and T.J. Martínez, J. Chem. Theo. Comp., 5, 1004-1015 (2009).
4) "First Principles Dynamics and Minimum Energy Pathways for Mechanochemical Ring-Opening of Cyclobutene," M.T. Ong, J. Leiding, H. Tao, A.M. Virshup, and T.J. Martínez, J. Amer. Chem. Soc., 131, 6377-6379 (2009).
5) "Graphical Processing Units for Quantum Chemistry," I.S. Ufimtsev and T.J. Martínez, Comp. in Sci. Eng., 10, 26-34 (2008).
6) "Electrostatic Control of Photoisomerization in the Photoactive Yellow Protein Chromophore: Ab Initio Multiple Spawning Dynamics," C. Ko, A. Virshup, and T.J. Martínez, Chem. Phys. Lett., 460, 272-277 (2008).
7) "Conformationally controlled chemistry: Excited state dynamics dictate ground state dissociation," M.H. Kim, L. Shen, H. Tao, T.J. Martínez, and A.G. Suits, Science, 315, 1561 (2007).
8) "QTPIE: Charge Transfer with Polarization Current Equalization. A fluctuating charge model with correct asymptotics," J. Chen and T.J. Martínez, Chem. Phys. Lett., 438, 315 (2007).
9) "Isomerization Through Conical Intersections," B.G. Levine and T.J. Martínez, Ann. Rev. Phys. Chem., 58, 613 (2007).
10) "Insights for Light-Driven Molecular Devices from Ab Initio Multiple Spawning Excited-State Dynamics of Organic and Biological Chromophores," T.J. Martínez, Acc. Chem. Res., 39, 119-126 (2006).
11) "Using Meta-Conjugation to Enhance Charge Separation versus Charge Recombination in Phenylacetylene Donor-Bridge-Acceptor Complexes," A.L. Thompson, T.-S. Ahn, K. R.J. Thomas, S. Thayumanavan, T.J. Martínez, and C.J. Bardeen, J. Amer. Chem. Soc., 127, 16348-16349 (2005).