Quantum Chemical Dynamics Research Articles

Synthesis of Hidden Subgroup Quantum Algorithms and Quantum Chemical Dynamics.

We describe a general formalism for quantum dynamics and show how this formalism subsumes several quantum algorithms, including the Deutsch, Deutsch-Jozsa, Bernstein-Vazirani, Simon, and Shor algorithms as well as the conventional approach to quantum dynamics based on tensor networks. The common framework exposes similarities among quantum algorithms and natural quantum phenomena: we illustrate this connection by showing how the correlated behavior of protons in water wire systems that are common in many biological and materials systems parallels the structure of Shor's algorithm.

Graph-Theoretic Molecular Fragmentation for Potential Surfaces Leads Naturally to a Tensor Network Form and Allows Accurate and Efficient Quantum Nuclear Dynamics.

Molecular fragmentation methods have revolutionized quantum chemistry. Here, we use a graph-theoretically generated molecular fragmentation method, to obtain accurate and efficient representations for multidimensional potential energy surfaces and the quantum time-evolution operator, which plays a critical role in quantum chemical dynamics. In doing so, we find that the graph-theoretic fragmentation approach naturally reduces the potential portion of the time-evolution operator into a tensor network that contains a stream of coupled lower-dimensional propagation steps to potentially achieve quantum dynamics with reduced complexity. Furthermore, the fragmentation approach used here has previously been shown to allow accurate and efficient computation of post-Hartree-Fock electronic potential energy surfaces, which in many cases has been shown to be at density functional theory cost. Thus, by combining the advantages of molecular fragmentation with the tensor network formalism, the approach yields an on-the-fly quantum dynamics scheme where both the electronic potential calculation and nuclear propagation portion are enormously simplified through a single stroke. The method is demonstrated by computing approximations to the propagator and to potential surfaces for a set of coupled nuclear dimensions within a protonated water wire problem exhibiting the Grotthuss mechanism of proton transport. In all cases, our approach has been shown to reduce the complexity of representing the quantum propagator, and by extension action of the propagator on an initial wavepacket, by several orders, with minimal loss in accuracy.

Atomic Dynamics From Coupled Quantized Electronic Oscillators

The quantum mechanical concept of an ”atom in a molecule" is synthetically emulated and analyzed in the form of a nonlinear electronic circuit. The circuit is comprised of coupled quantized oscillators. Quantized oscillators are novel and not generally known. Such systems are shown to be consistent with quantum chemical dynamics. Such a network of quantized oscillators achieves equilibriums that are consistent with naturally occurring molecules. These capabilities offer an alternative to von Neumann digital based computations, whereby solutions to complex systems are determined as equilibriums, as is the case of nature. Historically, quantized solutions have been associated with solutions to Shrödinger’s Equation. Shrödinger Equation based observations are typically rendered, if possible, by numerical calculations. By comparison, the approach presented in this work relies on synthesizing quantized electronic devices, which are novel, in order to achieve dynamical interactions consistent with naturally occurring ones. Detailed dynamics based on actual devices are provided. The theoretical underpinnings are described. This topic has broad implications for a variety of concepts associated with quantum mechanics, including computations and entanglement, as well as various oscillator circuits of fundamental importance.

Future directions of chemical theory and computation

Abstract Theoretical and computational chemistry aims to develop chemical theory and to apply numerical computation and simulation to reveal the mechanism behind complex chemical phenomena via quantum theory and statistical mechanics. Computation is the third pillar of scientific research together with theory and experiment. Computation enables scientists to test, discover, and build models/theories of the corresponding chemical phenomena. Theoretical and computational chemistry has been advanced to a new era due to the development of high-performance computational facilities and artificial intelligence approaches. The tendency to merge electronic structural theory with quantum chemical dynamics and statistical mechanics is of increasing interest because of the rapid development of on-the-fly dynamic simulations for complex systems plus low-scaling electronic structural theory. Another challenging issue lies in the transition from order to disorder, from thermodynamics to dynamics, and from equilibrium to non-equilibrium. Despite an increasingly rapid emergence of advances in computational power, detailed criteria for databases, effective data sharing strategies, and deep learning workflows have yet to be developed. Here, we outline some challenges and limitations of the current artificial intelligence approaches with an outlook on the potential future directions for chemistry in the big data era.

Decoherence speed limit in the spin-deformed boson model

In this paper, we study the role of the nonlinear environment on the bound passage time of dynamical quantum spin systems, which is of great interest in quantum control and has been applied to quantum metrology, quantum computation, and quantum chemical dynamics. We consider the decoherence speed limit for the spin-deformed bosonic model and the impacts of the nonlinear environment and its temperature on the decoherence speed limit. Moreover, we show that, at an early enough time, the parameters associated with the nonlinear environment exhibit important roles in controlling the decoherence process. In addition our results reveal that, in long times, these parameters do not affect the decoherence process.

Preface

The IUPAC Conference on Physical Organic Chemistry (ICPOC) series of biennial conferences has a long history as the leading international meeting on physical organic chemistry. From its first installment in Crans sur Sierre (Switzerland) in 1972, ICPOC has acted as a focus point for the physical organic community worldwide, and the conference series enjoys a respected international reputation. With its focus on relating chemical behavior and properties to molecular structure through the development of (ideally quantitative) understanding of structure–property relationships, physical organic chemistry (POC) finds wide application in tackling current scientific challenges and has been undergoing something of a resurgence in recent years. ICPOC-21, held on 9-13 September 2012 at the University of Durham, UK, provided a forum for researchers (329 delegates from 38 countries) in academia and industry, and at all career stages, to present their results to the POC community and to exchange ideas, meet old friends, and make new contacts while enjoying spectacular views of the World Heritage castle and cathedral. In line with a developing interpretation of POC as a widely applicable approach to chemistry, the scientific program embraced three broad themes (physical underpinnings, mechanism and catalysis, and supramolecular chemistry) but often these strands were as inseparably interconnected as the three leaves held in unity in the minimal saddle trefoil adopted as the conference logo. The scope of the meeting is illustrated by the selection of contributions included in this issue of Pure and Applied Chemistry. In addition to the 24 plenary and keynote lectures, there were 120 contributed talks and 141 poster presentations. Donna Blackmond's paper on the interplay of thermodynamics and kinetics in dictating organocatalytic reactivity and selectivity demonstrates the use of kinetic modeling to provide mechanistic understanding leading to practical application. Martin Tanner's discussion of rearrangements catalyzed by indole alkaloid prenyltransferases shows how the experimental POC approach may shed light on enzyme mechanisms, while Dean Tantillo describes the application of quantum chemical dynamics calculations to mechanistic problems in the field of terpene biosynthesis, and a blend of experiment and computation is presented in Brian Ohta's account of the structure of halonium ions in superacidic solutions. Hiromitsu Maeda reviews research on various stimuli-responsive circularly polarized luminescence properties derived from π-conjugated molecules and related materials, and Jason Harper summarizes progress towards a predictive understanding of how ionic solvents affect and control organic reactivity. Finally, Izumi Iwakura reports directly observations of the Claisen rearrangement by timeresolved vibrational spectroscopy using a few-optical-cycle pulse laser. This selection of excellent work provides only a taste, but the conference as a whole showed that the international POC community is as vital and vibrant as it has ever been, promising exciting times ahead for this approach to chemistry. The next ICPOC will be held in Ottawa (Canada) 10-15 August 2014 under the chairmanship of Prof. Paul Mayer (University of Ottawa). Further information can be found at http://events.science.uottawa. ca/icpoc22/welcome.html.Ian H. Williams Conference Chair and Editor

Quantum Speed Limits in Open System Dynamics

Bounds to the speed of evolution of a quantum system are of fundamental interest in quantum metrology, quantum chemical dynamics, and quantum computation. We derive a time-energy uncertainty relation for open quantum systems undergoing a general, completely positive, and trace preserving evolution which provides a bound to the quantum speed limit. When the evolution is of the Lindblad form, the bound is analogous to the Mandelstam-Tamm relation which applies in the unitary case, with the role of the Hamiltonian being played by the adjoint of the generator of the dynamical semigroup. The utility of the new bound is exemplified in different scenarios, ranging from the estimation of the passage time to the determination of precision limits for quantum metrology in the presence of dephasing noise.

Quantum chemical dynamics in two dimensions

An instanton theory for finding both tunnel splittings and incoherent tunneling rates in two-dimensional potentials is presented. The exact two-dimensional extremal trajectory is numerically calculated, and then the prefactor is expressed via its stability parameter. The method does not require any specific form of the potential and permits one to describe the rate constant at temperatures ranging from that of the low-temperature plateau to the classical activated transitions. The symmetric double well and the metastable well coupled to vibration are studied as examples, and, for the latter case, the instanton results are compared with recent numerical complex scaling method data, showing excellent agreement. The method is applied to inter- and intramolecular hydrogen transfer reactions. It is shown that in the latter case (tunnel splitting in malonaldehyde and in hydrogenoxalate anion) the tunneling trajectory is far from both the cutting-corner straight line and from the minimum energy path.