Publications

Disclaimer: ... but to those whom God has called, both Jews and Greeks, Christ the power of God and the wisdom of God. For the foolishness of God is wiser than man's wisdom, and the weakness of God is stronger than man's strength. - 1 Corinthians 1:24-31 (NIV)

Dynamics of open-shell species at metal surfaces S. Roy, N. Shenvi, J. C. Tully J. Phys. Chem. C 113, 16311-16320, 2009. Synopsis: The challenges of simulating the dynamics of open shell systems near metal surfaces are reviewed. We present some of our recent results for the vibrational relaxation of NO on Au(111).
The initial and final states of electron and energy transfer processes: Diabatization as motivated by system-solvent interactions J. E. Subotnik, R. J. Cave, R. S. Steele, and Neil Shenvi J. Chem. Phys 130, 234102, 2009. Synopsis: We show how diabatic states for charge or energy transfer in solution can be constructed using Boys and Edmiston-Ruedenberg (ER) formalisms. We show that both of these schemes can be derived through an appropriate assumption about solvent-solute interaction. For the case of oligomethylphenyl-3, we show that ER diabatic states can be used to visualize localized electron-hole pairs.
Nonadiabatic scattering at metal surfaces: independent-electron surface hopping Neil Shenvi, S. Roy, and J. C. Tully J. Chem. Phys 130, 174107, 2009. Synopsis: We construct an independent-electron surface hopping (IESH) algorithm which allows the simulation of mixed quantum-classical nonadiabatic dynamics of adsorbates on metal surfaces. Our algorithm captures the effects of the electronic continuum on the metal and the excitation of multiple electron hole pairs. Taking advantage of the fact that electrons are independent, we are able to simulate nonadiabatic molecular dynamics in a Hilbert space of >100 billion adiabatic electronic states, a feat which would be impossible using traditional surface hopping methods.
Model Hamiltonian for the interaction of NO with the Au(111) surface Sharani Roy, Neil A. Shenvi and J.C. Tully J. Chem. Phys 130, 174716, 2009. Synopsis: Using density functional theory (DFT) calculations, we construct a model Hamiltonian for the interaction of NO with the Au(111) surface. The model Hamiltonian is a 2x2 matrix representing the neutral and ionic electronic states of the NO molecule. By fitting these diabatic states to an analytic functional form, we obtain a good fit to the ab initio ground state energies and charges on the NO molecule. Adiabatic MD simulations using our model reproduce several qualitative results of NO/Au(111) scattering experiments.
Phase-space surface hopping: Nonadiabatic dynamics in a superadiabatic basis. N. Shenvi J. Chem. Phys 130, 124117, 2009. Synopsis: We construct a superadiabatic basis in phase-space such that the electronic state is a function of both the nuclear position and momenta in analogy to Berry's first-order superadiabatic basis. We extend the fewest-switches surface hopping algorithm to phase-space within this framework.
Transition state barriers in multidimensional Marcus theory. J. Zwickl, N. Shenvi, J.R. Schmidt, and J. C. Tully J. Phys. Chem. A 112, 10570, 2008. Synopsis: We show that the use of a single reaction coordinate to describe transfer of multiple particles in multidimensional Marcus theory is exact within limit of linear solvent response. If solvent response is nonlinear, a single coordinate is shown to still work well in practice.
Controlling spin contamination using constrained density functional theory. J.R. Schmidt, Neil Shenvi, and John C. Tully J. Chem. Phys.,129, 114110, 2008. Synopsis: We show that constrained DFT can be used to minimize spin contamination in unrestricted calculations. The use of local constraints also allows us to calculate hyperfine coupling values for a transition metal complex and to construct diabatic potential energy curves for the dissociation of OF radical.
Efficient discretization of the continuum through complex contour deformation. Neil Shenvi, J.R. Schmidt, Stephen T. Edwards, and John C. Tully Phys. Rev. A,78, 022502, 2008. Synopsis: We show that a continuum of states can be efficiently represented by a discrete set of states using complex contour deformation and Gaussian quadrature. Our method, which can be viewed as a type of "complex scaling", has applications for adatom-metal scattering simulations which require careful and expensive treatment of the metal continuum.
Recursively generated linear constraints for variational two-particle reduced-density-matrix theory. Tamas Juhasz, Neil Shenvi, and David A. Mazziotti Chem. Phys. Lett., 445, 79-83, 2007. Synopsis: We generate recursive constraints which implement partial 3-positivity conditions on a reduced density matrix calculations of molecular ground-state energies. The ground state energy is shown to converge to the correct ground state with a high degree of accuracy and only a limited number of iterations.
Semiclassical Dynamics of Electron Transfer at Metal Surfaces. Hongzhi Cheng, Neil Shenvi, and John Tully Phys. Rev. Lett., 99, 053201, 2007. Synopsis: We apply semiclassical dynamics based on Gaussian propagation to the simulation of electron transfer events of ions scattering from a metal surface. The semiclassical method is very accurate, but is substantially faster than full quantum dynamics.
Nonadiabatic dynamics near metal surfaces: decoupling quantum equations of motion in the wideband limit. Neil Shenvi, Hongzhi Cheng, John Tully Phys. Rev. A, 74, 062902, 2006. Synopsis: we rederive the local complex potential method for simulating coupled nuclear-electronic dynamics in the presence of an electronic continuum. By decoupling the electronic equations of motion, we can simulate the full quantum nuclear dynamics at a fraction of the cost of traditional methods.
Vibrational relaxation of NO on Au(111) via electron-hole pair generation. Neil Shenvi, Sharani Roy, Priya Parandekar, John Tully J. Chem. Phys., 125, 154703, 2006. Synopsis: we propose a model for vibrational relaxation of NO molecules when scattered from a Au(111) surface, following the experimental work of Wodtke et al. In our model, relaxation occurs by Franck-Condon mediated transitions between the NO and NO- diabats, generating hot electrons in the metal.
Expectation value constraints for the N-representability problem. N. Shenvi, and K.B. Whaley. Phys. Rev. A, 74, 022507 (2006). Synopsis: We derive upper bounds on the expectation values of N-representable bosonic and fermionic density matrices with arbitrary 2-particle geminals. These bounds could potentially be used to improve the accuracy of density matrix variational ground state energy calculations.
Topics in Quantum Computation N. Shenvi's PhD Dissertation. Includes chapters on quantum random walks, oracle noise in Grover's algorithm, electron spin decoherence, and the N-representability problem.
Qubit coherence control in a nuclear spin bath. R. de Sousa, N. Shenvi, and K.B. Whaley. Phys. Rev. B, 72, 045330, (2005). Synopsis: we develop a formalism for treating decoherence of an electron spin in a spin-I nuclear lattice due to nuclear spectral diffusion. We calculate the scaling of the CPMG sequence time as a function of I.
Universal scaling of hyperfine-induced electron spin echo decay. N. Shenvi, R. de Sousa, and K.B. Whaley. Phys. Rev. B, 71, 224411, (2005). Synopsis: we provide numerical evidence that the decoherence caused by the contact hyperfine interaction between a localized electron and a lattice of nuclear spins can be substantially removed by a single spin echo pulse sequence. We further demonstrate that the residual visibility loss after the spin echo pulse sequence scales as 1/B^2.
Nonperturbative bounds on electron spin coherence times induced by hyperfine interactions. N. Shenvi, R. de Sousa, and K.B. Whaley. Phys. Rev. B, 71, 144419, (2005). Synopsis: we derive rigorous, analytic bounds on the T1 and T2 relaxation times of a localized electron spin interacting with a lattice of nuclear spins via the hyperfine interaction. We show that above some critical field Bc, the longitudinal relaxation time T1 is infinite while a finite, non-trivial lower bound is given for the transverse relaxation time T2.
Transmission spectrum of an optical cavity containing N atoms. S. Leslie, N. Shenvi, K.R. Brown, D.M. Stampur-Kurn and K.B. Whaley. Phys. Rev. A, 69, 043805, (2004). Synopsis: we calculate the transmission spectrum of a high-finesse optical cavity containing N atoms in an arbitrarily loose harmonic trap. We show that in the limit of a tight-trap we recover the Lambe-Dicke limit, while in the loose trap limit, we obtain a semi-classical result.
Effects of a random noisy oracle on search algorithm complexity. N. Shenvi, K.R. Brown and K.B. Whaley. Phys. Rev. A, 68, 052313, (2003). Synopsis: we show that random oracle phase errors in a standard implementation of Grover's algorithm lead to a loss of speed-up over the classical search above some threshhold noise level.
Quantum random-walk search algorithm. N. Shenvi, J. Kempe and K.B. Whaley. Phys. Rev. A, 67, 052307, (2003). Synopsis: we demonstrate that a quantum search algorithm can be constructed using a quantum random walk formalism. We prove that such an algorithm provides an O(sqrt(N)) speed-up over the classical algorithm.
Efficient chemical kinetic modeling through neural network maps. Neil Shenvi, J.M. Geremia, and Herschel Rabitz. J. Chem. Phys., 120, 9942, 2004. Synopsis: we use ridge-polynomial neural networks to model the chemical kinetics of a variety of complex chemical reactions. The neural networks can be efficiently evaluated and are highly accurate, making them useful in a variety of applications which require the repeated integration of chemical kinetic equations.
Substituent Ordering and Interpolation in Molecular Library Optimization. Neil Shenvi, J.M. Geremia, and Herschel Rabitz. J. Phys. Chem. A, 107(12), 2066-2074, 2003. Synopsis: we demonstrate that by correctly ordering the substituents in a combinatorial library, properties of interest can be predicted from experimental data even when the experimental library sampling is very sparse. The substituents can be correctly ordered using a genetic algorithm without any knowledge of the specific structure or properties of the substituents.
Nonlinear Kinetic Parameter Identifications through Map Inversion. Neil Shenvi, J.M. Geremia, and Herschel Rabitz. J. Phys. Chem. A, 106(51), 12315-12323, 2002. Synopsis: we use HMDR (High Dimensional Model Representation) to generate simple maps which approximate complex high-dimensional functions. These maps are then used to invert chemical kinetic data and obtain the rate constants consistent with the data.

Talks

The Weirdness of Quantum Mechanics

Abstract: Quantum mechanics replaced classical mechanics almost a century ago as the dominant theory of microscopic physics. Unlike classical Newtonian mechanics, which is in many ways intuitive and ordinary, quantum mechanics is replete with inherent weirdness. In the last decade, it has been demonstrated that the unique properties of quantum objects have practical applications in such fields as spintronics, quantum cryptography and quantum computation. Furthermore, the basic postulates of quantum mechanics raise important philosophical questions about measurability, determinism, and the nature of reality. This talk will outline the fundamental postulates of quantum mechanics, their practical utility, and their philosophical implications.

Introduction to Quantum Computation

Abstract: The field of quantum computation is intrinsically multi-disciplinary, due to the theoretical and practical obstacles to building a quantum computer. In this talk, I give an introduction to the theoretical underpinnings of quantum computation, touching on topics in discrete mathematics, classical computer science, and solid-state physics. I also provide an overview of the circuit model of quantum computation and explain common notation.