Abstracts

 The abstracts are posted when received. Please come back to this page. If you are presenting, please send your abstract as soon as possible to Salomon Billeter. Thank you.

Oleg Prezhdo

Incorporating quantum solvent effects into non-adiabatic molecular dynamics with application to ultrafast electron transfer from an organic donor to a semiconductor acceptor

Several recent approaches dealing with quantum effects in condensed phase environments will be discussed. Standard non-adiabatic molecular dynamics (NA MD) treats NA transitions in a small subsystem quantum mechanically and the majority of the degrees of freedom classically. The techniques developed in our group modify NA MD to include quantum effects for all degrees of freedom, while still allowing for application of the modified NA MD to condensed phase systems. The following approaches will be discussed. 1. The stochastic mean-field approximation [1,2] incorporates quantum decoherence into the quantum-classical mean-field and disposes of the ad hoc surface hopping ansatz. 2. NA MD based on the Bohmian formulation of quantum mechanics [3] resolves the quantum backreaction branching problem and allows for a greater flexibility in the coupling of quantum and classical subsystems. 3. Quantized Hamilton dynamics [4-7], including the quantized mean-field approach [5] incorporates zero point energy and tunneling effects by a straightforward extension of classical mechanics into quantum dimensions. Application of standard NA MD in combination with density functional theory to ultrafast photo-induced electron transfer from a molecular electron donor to TiO2 acceptor will be presented [8]. The transfer process forms the foundation for the solar cells of the Graetzel type and is typical of the dye sensitized semiconductor nanomaterials used in photocatalysis and photoelectrolysis.
  1. O. V. Prezhdo, "Mean field approximation for the stochastic Schrodinger equation", J. Chem. Phys. 111, p.8366 (1999)
  2. O. V. Prezhdo, "Quantum anti-Zeno acceleration of a chemical reaction", Phys. Rev. Lett. 85, p.4413 (2000)
  3. O. V. Prezhdo, C.Brooksby, "Quantum backreaction through the Bohmian particle", Phys. Rev. Lett. 86, p.3215 (2001)
  4. O. V. Prezhdo, Yu. V. Pereverzev, "Quantized Hamilton dynamics", J. Chem. Phys. 113, p.6557 (2000)
  5. C. Brooksby, O. V. Prezhdo, "Quantized mean-field approximation", Chem. Phys. Lett. 346 p.463 (2001)
  6. O. V. Prezhdo, Yu. V. Pereverzev, "Quantized Hamilton dynamics for a general potential", J. Chem. Phys., in press
  7. E. Pahl, O. V. Prezhdo, "Extension of quantized Hamilton dynamics to higher orders", J. Chem. Phys., in the review process
  8. W. Stier and O. V. Prezhdo, "Non-adiabatic molecular dynamics simulation of light-induced electron transfer from an anchored molecular electron donor to a semiconductor acceptor", J. Phys. Chem. B, in press

William H. Miller

Using the Semiclassical Initial Value Representation to Add Quantum Effects to Classical Molecular Dynamics Simulations

Semiclassical (SC) theory provides a way for adding the quantum mechanical effects of interference and tunneling to classical mechanics (to a good level of approximation), and the initial value representation is a practical way of implementing SC theory for systems with many degrees of freedom. Applications of these approaches to several models of complex systems, involving hundreds of degrees of freedom, will be presented.

Nikos Doltsinis

Non-adiabatic Car-Parrinello molecular dynamics

An extension of Car-Parrinello molecular dynamics for efficient treatment of electronically nonadiabatic transitions is presented. The current approach couples the S1 restricted open-shell Kohn-Sham excited state to the S0 ground state using Tully's fewest switches surface hopping algorithm. Efficient evaluation of the nonadiabatic couplings is achieved by exploiting the available wavefunction time derivatives. As a demonstration, we have investigated the cis-trans photoisomerization of formaldimine, the minimal model of a Schiff base. The method makes possible dynamical ab initio simulations beyond the Born-Oppenheimer approximation of systems similar in character and complexity as those typically studied by standard Car-Parrinello methods. Since the computational cost scales linearly with the number of excited states, the technique is ideally suited to study the photochemistry of complex molecules, particularly in condensed phases.

David Coker

Modeling charge transfer excited state dynamics in solution

We explore the implementation of semi-empirical methods such as diatomics-in-ionic-systems (DIIS) and diatomics-in-molecules (DIM) for computing accurate poly-atomic potential surfaces. These methods are implemented with in both the surface hopping and mapping hamiltonian frameworks to explore mixed quantum-classical and semiclassical descriptions of charge transfer to solvent processes in the ion pair states of I2 in liquid and solid xenon.

Koji Ando

Solvent nuclear and electronic quantum effects in electron transfer reactions

In the recent MD simulation study on aqueous electron transfer (ET), we found it essential to take account of the solvent electronic polarization effects to the reorganization energy at least via an empirical scaling correction in order to reproduce the experimental kinetic isotope effect. On the other hand, the quantum rate enhancement factor was found to vary by an order of magnitude depending on the choice of the correction parameter of the optical dielectric constant (while the isotope ratio was rather robust). It is thus vital to go beyond the empirical scaling correction toward more realistic modelling of the electronic polarization effects in ETs.
To this end, we have implemented (with modifications) the dynamical fluctuating charge (fluc-q) model into our ET simulation code. It was found that the solvent reorganization energy is renormalized, both in the magnitude and in the slope along the inverse donor-acceptor distance. In the time-correlation and spectral density functions of the solvent reaction coordinate, the frequency of the librational coupling motion is blue-shifted and its intensity is suppressed due to the solvent electronic polarization. Its impact on the scaled quantum energy gap law of the ET rate was found to be modest.

Eberhard K.U. Gross

Density functional theory of the combined system of electrons and nuclei

Sharon Hammes-Schiffer

Multiconfigurational nuclear-electronic molecular orbital approach

A nuclear-electronic orbital (NEO) method for the simultaneous calculation of electronic and nuclear wavefunctions will be presented. Both electronic and nuclear molecular orbitals are expressed as linear combinations of Gaussian basis functions, and the variational method is utilized to minimize the energy with respect to all molecular orbitals. The centers of the nuclear basis functions are also optimized variationally. Significant correlation effects are included using a multiconfigurational self-consistent-field (MCSCF) approach. Analytic gradients allow for geometry optimizations and for the generation of minimum energy paths and dynamic reaction paths. Applications to hydrogen transfer reactions will be presented. For hydrogen transfer reactions, the transferring hydrogen nuclei, as well as all electrons, are treated quantum mechanically to include nuclear quantum effects such as zero point energy and hydrogen tunneling.

Uwe Manthe

Quantum dynamics of non-adiabatic processes: System-bath separation and mixed quantum-classical dynamics

Quantum dynamics calculations of condensed phase systems require a separation of the complete system: only a inner system of limited size can be described employing an accurate quantum treatment. The remaining system, the "bath", has to be described by approximate models or classical mechanics. Within this approach, the theoretical treatment of the coupling between the two subsystems is a key problem.

Two approaches addressing this problem will be discussed: reduced density matrix theory based on a perturbative treatment of the system-bath coupling and a mixed quantum-classical Liouville approach utilizing a surface hopping scheme. As an example, the non-adiabatic dynamics in pyrazine is studied.

James T. Hynes

On the theory of the dissociation of aromatic radical anions in solution: A ground electronic state conical intersection problem

Damien Laage(a), Irene Burghardt (b), Thomas Sommerfeld (c), James T.Hynes (a,d)

a) UMR 8640 PASTEUR, Ecole Normale Superieure, Paris, France
b) UMR 8642, Ecole Normale Superieure, Paris, France
c) Physikalisch-Chemisches Institut, Heidelberg, Germany
d) Dept of Chemistry and Biochemistry, Univ. of Colorado, Boulder, USA

Conical intersections (two electronic states which `cross' as a function of a certain coordinate but whose coupling is only finite--such that an avoided crossing is produced--due to motion in a second coordinate) have attracted considerable recent attention as `funnels' for nonadiabatic transitions from excited electronic states to ground electronic states. There has however been almost no attention given to the influence of conical intersections on ground electronic state chemical reactions. Here we present what is evidently the first theoretical study of such a phenomenon. In particular, we will describe a theoretical study of the thermal unimolecular dissociation of aromatic radical anions in solution:

[ArX].- ----> Ar. + X-

These reaction systems are important in connection with the SRN1 reaction mechanism, significant both in synthesis and fundamental questions of nucleophile--electrophile chemistry, and arguably are important for the mechanism of the Grignard reaction, of central synthetic interest. Further, closely related molecules (halouracils) and reactions may be involved in DNA damage induced by ionizing radiation.

We present the theoretical formulation and calculation of reaction rates and reaction paths for a paradigm radical anion [CN-phi-Cl].-, where phi denotes a phenyl ring, in polar solvents. The theory constructed involves specialized electronic structure calculations and a nonequilibrium solvation description at the dielectric continuum level. It is novel in a number of ways, most notably in the critical feature that a conical intersection is involved. The dissociation is only allowed by an electronic coupling--between a pi*electronic state where the charge is localized in the aromatic ring system, and a sigma*state key for the C-Cl bond breaking--and this coupling is only finite for out of plane wagging of that bond coordinate. Comparison with experiments is made, and the agreement is generally encouraging, especially given the fact that no adjustment of any parameter in the theory was made to `force' any agreement.

While the specific reaction studied turns out be completely electronically adiabatic due to the large electronic coupling involved, we will sketch out some possible situations where explicit attention to nonadiabatic dynamics will be required. We will also describe how the above analytic treatment could be converted to a simulation treatment.

Gabriel G. Balint-Kurti

Time-Dependent Quantum Theory applied to electronically non-adiabatic dynamics of small molecules

Time-dependent quantum theory of reactive scattering and molecular photodissociation will be reviewed [1] and examples of recent work will be given. In particular new advances in the field, which permit the use of real, as opposed to complex, algebra to be used and which lead to significant enhancement of computational efficiency will be outlined [2]. The use of time-dependent quantum wavepacket dynamics in the study of electronically non-adiabatic processes will be discussed [3,4]. Examples of both electronically adiabatic and non-adiabatic processes will be given and will include the reactive scattering processes

O + H2(j) --> OH + H ; O + HCl --> OH + Cl and O + HCl --> OCl + H [3,5]

and the photodissociation processes

HF + hn --> F*/F + H ; HOBr + hn --> OH + Br and N2O + hn --> N2 + O [4,5,6].

Branching fraction G=s*/(s+s*) from the Photodissociation of HF(v=3). The structure shown in the figure arises from the nodal structure in the initial vibrationally excited wavefunction. The figure shows the only experimental measurement of the branching fraction for HF and demonstrates that our ab initio calculations are in good agreement with the only available experimental measurement. Calculated and experimental cross section ratios for O+H2(j=1) and O+H2(j=0)
  1. G.G. Balint-Kurti, R.N. Dixon and C.C. Marston, Internat. Rev. Phys. Chem., 11, 317 (1992).
  2. S.K. Gray and G.G. Balint-Kurti, J. Chem. Phys., 108, 950 (1998).
  3. S. K. Gray, G. G. Balint-Kurti, G. C. Schatz, et al., J. Chem. Phys., 113, 7330 (2000).
  4. A. Brown and G.G. Balint-Kurti, J. Chem. Phys., 113, 1870 (2000); 113, 1879 (2000).
  5. V. Piermarini, G.G. Balint-Kurti, et al., J. Phys. Chem.A, 105, 5743 (2001).
  6. G.G. Balint-Kurti, L. Füsti-Molnàr and A. Brown, Phys. Chem. Chem. Phys., 3, 702 (2001).

Peter Rossky

Computationally convenient description of electronic coherence evolution in mixed quantum-classical condensed phase simulations

In general, the rate of decay of a non-stationary electronic state will be influenced by the coherence among the amplitudes for the initial state and the component decay channels. For a species in a condensed phase, the dynamics of the bath can strongly dissipate this coherence and thus modify the rate of electronic evolution. A rigorous estimator of the rate of electronic state decoherence which can be readily evaluated from the results of simulations using classical molecular descriptions will be described and applied to evaluate the dissipative role of the environment. Applications illustrating the remarkably rapid rate of decoherence will be discussed, including electronic excited state evolution in solution and biological electron transfer. Alternative computational implementations for solvated molecular electronic states will be illustrated.

Salomon Billeter

Hybrid approach for including electronic and nuclear quantum effects in molecular dynamics simulations of hydrogen transfer reactions in enzymes and other systems

A hybrid approach for simulating proton and hydride transfer reactions in enzymes is presented [1]. The electronic quantum effects are incorporated with an empirical valence bond model. The nuclear quantum effects of the transferring hydrogen are included with a mixed quantum/classical molecular dynamics method in which the hydrogen nucleus is described as a multidimensional vibrational wavefunction.

The free energy profiles are obtained as functions of a collective reaction coordinate. A perturbation formula is derived to incorporate the vibrationally adiabatic nuclear quantum effects into the free energy profiles. The dynamical effects are studied with the molecular dynamics with quantum transitions (MDQT) surface hopping method, which incorporates nonadiabatic transitions among the adiabatic hydrogen vibrational states. The MDQT method is combined with a reactive flux approach to calculate the transmission coefficient and to investigate the real-time dynamics of reactive trajectories.

This hybrid approach includes nuclear quantum effects such as zero point energy, hydrogen tunneling, and excited vibrational states, as well as the dynamics of the complete enzyme and solvent. The nuclear quantum effects are incorporated during the generation of the free energy profiles and dynamical trajectories rather than subsequently added as corrections. Moreover, this methodology provides detailed mechanistic information at the molecular level and allows the calculation of rates and kinetic isotope effects. An initial application of this approach to the enzyme liver alcohol dehydrogenase (LADH) is also presented [2].

The range of applications of this approach is not limited to hydrogen transfer reactions in enzymes. Perspectives and problems in using this approach for other types of systems will be discussed.

  1. S.R. Billeter, S.P. Webb, T. Iordanov, P.K. Agarwal, and S. Hammes-Schiffer, "Hybrid approach for including electronic and nuclear quantum effects in molecular dynamics simulations of hydrogen transfer reactions in enzymes", J. Chem. Phys. 114 (2001) 6925.
  2. S.R. Billeter, S.P. Webb, P.K. Agarwal, T. Iordanov, and S. Hammes-Schiffer, "Hydride transfer in liver alcohol dehydrogenase: Quantum dynamics, kinetic isotope effects, and role of enzyme motion", J. Am. Chem. Soc. 123 (2001), 11262.

Raymond E. Kapral

Statistical Mechanics of Quantum-Classical Systems

The time evolution of many condensed phase systems can be described in terms of non-adiabatic transitions among quantum states as a result of coupling to a classical environment. In such circumstances our primary interest is often in the values of observables such as transport properties determined from equilibrium time correlation functions. Consequently, one must consider the statistical mechanics of systems governed by quantum-classical dynamics and not simply time their evolution. The talk will focus on the development of such a statistical mechanical theory. Quantum-classical time evolution will be compared to that of full quantum mechanics. Given that a system obeys quantum-classical dynamics, linear response theory will be used to obtain expressions for time correlation functions and transport properties. The forms of quantum-classical time correlation functions and and the equilibrium density will be discussed. The results provide a framework and specify an algorithm for the computation of equilibrium time correlation functions and transport properties for mixed quantum-classical systems.
  1. S. Nielsen, R. Kapral and G. Ciccotti, J. Chem. Phys. 115, 5805 (2001).

Michael Thoss

Systematic convergence in the dynamical hybrid approach: A method to describe quantum dynamics in dissipative systems

Michael Thoss(a) and Haobin Wang(b)

a) Theoretical Chemistry, Technical University of Munich, D-85747 Garching, Germany
b) Department of Chemistry and Biochemistry , New Mexico State University, Las Cruces, NM 88003, USA

A method for simulating quantum dynamics in dissipative systems is presented [1,2]. The method is based on an iterative convergence procedure for a dynamical hybrid approach. In this approach, the overall system is first partitioned into a ``core'' and a ``reservoir''. The former is treated via an accurate quantum mechanical method and the latter is treated via a more approximate method. Next, the number of ``core'' degrees of freedom is systematically increased to achieve numerical convergence for the overall quantum dynamics.

The method is applied to several examples of quantum dissipative dynamics in the condensed phase: the spin-boson problem with Debye spectral density as a model for electron-transfer reactions in polar solvents, electronic resonance decay in the presence of a vibrational bath, and ultrafast photoinduced electron-transfer reactions in mixed valence compounds [3].

  1. H. Wang, M. Thoss, and W.H. Miller, J. Chem. Phys. 115, 2979 (2001).
  2. M. Thoss, H. Wang, and W.H. Miller, J. Chem. Phys. 115, 2991 (2001).
  3. M. Thoss and H. Wang, Chem. Phys. Lett., submitted.

Craig C. Martens

Simulating Quantum Processes using Entangled Classical Trajectories

In this talk, I will describe a general approach to simulating quantum processes using classical trajectories. The method is based on solving the quantum Liouville equation approximately by using trajectory ensembles evolving in phase space. The formalism will be developed and applied to classical-limit molecular dynamics on coupled electronic surfaces and quantum tunneling through a potential barrier. In both applications, quantum effects emerge as a breakdown of the statistical independence of the members of the ensemble. This leads to nonclassical interaction between trajectories and entanglement of the evolving ensemble.

Todd J. Martínez

Ab Initio and "Nearly Ab Initio" Quantum Molecular Dynamics

Ab initio molecular dynamics (AIMD) methods solve the electronic Schrödinger equation `on-the-fly' in order to obtain the necessary potential energy surfaces and gradients. This avoids tedious fitting procedures which are necessary when parameterizing potential energy surfaces to analytic functional forms, and also allows bond rearrangements to be treated naturally. Although computationally challenging, the implementation of AIMD methods in the context of classical nuclear dynamics is conceptually straightforward. In contrast, there are fundamental conceptual problems with extending these ideas to cases where the nuclear dynamics is treated quantum mechanically, since the nuclear Schrödinger equation is not local in nuclear configuration space. We have recently developed and applied the ab initio multiple spawning (AIMS) method which reconciles the simultaneous solution of the nuclear and electronic Schrödinger equations. The methods and applications are described, in the context of dynamics on multiple electronic states and nuclear tunneling. We emphasize our studies of photoinduced cis-trans isomerization, which have proceeded from AIMS treatment of smaller paradigmatic molecules (such as ethylene and butadiene) through more traditional time-independent electronic structure studies of larger chromophores and photodynamical studies of proteins with parameterized potential energy surfaces. The results of these studies predict that charge-transfer is the key to efficient nonradiative decay in these processes and furthermore that the outcome and time scale of the excited state dynamics can be controlled by tuning the electrostatic environment. We also discuss preliminary results obtained using reparameterized semiempirical methods in place of ab initio electronic structure theory, with and without QM/MM methodologies.

Jürg Hutter

Excited State Energy Surfaces from Density Functional Theory

Progress in density functional based methods for excited states let hope for computationally efficient and accurate descriptions of excited state energy surfaces. The restricted open-shell Kohn-Sham (ROKS) method allows for the calculation of the first excited singlet state at a cost comparable to gound state calculations. The performance of the ROKS method will be discussed and extensions to overcome failiures of the method introduced. Finally, methods based on time-dependent density functional theory will be introduced and first studies on their applicability in dynamical simulations presented.

Giovanni Ciccotti

Wigner approach to quantum-classical nonadiabatic dynamics

We use partial Wigner representation of quantum dynamics to formulate quantum-classical evolution laws for a quantum subsystem coupled with a classical bath [1,2]. In this framework we will write down the quantum-classical Liouville equation and its counterpart for dynamical variables (as we will see, the evolution equations for the phase space variables are no more as fundamental as one could desire). Some unusual features of this dynamics will be also discussed. The evolution equation for the density matrix is expressed in an adiabatic basis while its solution can be determined in terms of an ensemble of surface-hopping trajectories. An analogous analysis provides the evolution of any dynamical variable. To give explicitly the flavor (and the difficulties) of the derived algorithm, we have solved the dynamics of the spin-boson model [3]. With reference to this very simple model (for which our quantum-classical dynamics is an exact scheme) we will discuss in detail the algorithm for the simulation of the dynamics and the simple Monte Carlo sampling used to evaluate the expectation values of observables.
  1. R.Kapral and G.Ciccotti, J.Chem.Phys., 110, 8919, (1999)
  2. S.Nielsen, R.Kapral, and G.Ciccotti, J.Chem.Phys., 112, 6543, (2000)
  3. D.Mac Kernan, G.Ciccotti and R.Kapral, J.Chem.Phys., 116, 2346,(2002)

Donal Mac Kernan

Long Time Propagators for Mixed Quantum Classical Dynamics

Quantum-classical dynamics describes systems composed of a quantum subsystem coupled to a classical environment and provides a means to study the dynamics of many-body systems that are not amenable to investigation using full quantum dynamics. In the quantum-classical approach we use [1,2], the isolated quantum subsystem and bath obey quantum mechanics and classical mechanics, respectively, but their coupled evolution is given by quantum-classical equations of motion where a simple Newtonian description of the environmental degrees of freedom no longer exists. In recent work, we showed that this approach reproduces essentially exact results for the spin-boson model [3], but the demonstration was limited to short and medium times. Currently, we are developing two types of propagators which we hope will be able to generate long time dynamics - one is based on an integral solution to the Dyson equation, while the other is based on a Trotter factorization of the propagator. In my contribution, I will explain how each of these propagators is obtained, their numerical implementation, and present the corresponding computational results to date.
  1. R. Kapral and G. Ciccotti, J. Chem. Phys. 110, 8919 (1999).
  2. S. Nielsen, R. Kapral and G. Ciccotti, J. Chem. Phys. 112, 6543 (2000).
  3. D. Mac Kernan, G. Ciccotti and R. Kapral, J. Chem. Phys. 116, 2346 (2002).
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SB, 03-11-2002