G. S. Ezra and S. Wiggins, Sampling Phase Space Dividing Surfaces Constructed from Normally Hyperbolic Invariant Manifolds (NHIMs), J. Phys. Chem. A., 122(42), 8354–8362 (2018).
In this paper, we further investigate the construction of a phase space dividing surface (DS) from a normally hyperbolic invariant manifold (NHIM) and the sampling procedure for the resulting dividing surface described in earlier work (Wiggins, S.; J. Chem. Phys. 2016, 144, 054107). Our discussion centers on the relationship between geometrical structures and dynamics for 2 and 3 degree of freedom (DoF) systems, specifically, the construction of a DS from a NHIM. We show that if the equation for the NHIM and associated DS is known (e.g., as obtained from Poincaré–Birkhoff normal form theory), then the numerical procedure described in Wiggins et al. ( J. Chem. Phys. 2016, 144, 054107) gives the same result as a sampling method based upon the explicit form of the NHIM. After describing the sampling procedure in a general context, it is applied to a quadratic Hamiltonian normal form near an index-one saddle where explicit formulas exist for both the NHIM and the DS. It is shown for both 2 and 3 DoF systems that a version of the general sampling procedure provides points on the analytically defined DS with the correct microcanonical density on the constant-energy DS. Excellent agreement is obtained between analytical and numerical averages of quadratic energy terms over the DS for a range of energies.
V. J. Garcia-Garrido, J. Curbelo, A. M. Mancho, S. Wiggins, and C. R. Mechoso, The Application of Lagrangian Descriptors to 3D Vector Fields, Regular and Chaotic Dynamics, 23(5), 551-568 (2018).
Since the 1980s, the application of concepts and ideas from Dynamical Systems Theory to analyze phase space structures has provided a fundamental framework to understand long-term evolution of trajectories in many physical systems. In this context, for the study of fluid transport and mixing the development of Lagrangian techniques that can capture the complex and rich dynamics of time dependent flows has been crucial. Many of these applications have been to atmospheric and oceanic flows in two-dimensional (2D) relevant scenarios. However, the geometrical structures that constitute the phase space structures in time dependent three-dimensional (3D) flows require further exploration. In this paper we explore the capability of Lagrangian Descriptors (LDs), a tool that has been successfully applied to time dependent 2D vector fields, to reveal phase space geometrical structures in 3D vector fields. In particular we show how LDs can be used to reveal phase space structures that govern and mediate phase space transport. We especially highlight the identification of Normally Hyperbolic Invariant Manifolds (NHIMs) and tori. We do this by applying this methodology to three specific dynamical systems: a 3D extension of the classical linear saddle system, a 3D extension of the classical Duffing system, and a geophysical fluid dynamics f-plane approximation model which is described by analytical wave solutions of the 3D Euler equations. We show that LDs successfully identify and recover the template of invariant manifolds that define the dynamics in phase space for these examples.
Article, “Influence of mass and potential energy surface geometry on roaming in Chesnavich’s CH4+ model,” was published 07 September 2018, in The Journal of Chemical Physics (Vol.149, Issue 9). Vladimir Krajnak and Stephen Wiggins
It may be accessed via the link below:
Chesnavich’s model Hamiltonian for the reaction CH+4 → CH+3 + H is known to exhibit a range of interesting dynamical phenomena including roaming. The model system consists of two parts: a rigid, symmetric top representing the CH+3 ion and a free H atom. We study roaming in this model with focus on the evolution of geometrical features of the invariant manifolds in phase space that govern roaming under variations of the mass of the free atom m and a parameter a that couples radial and angular motion. In addition, we establish an upper bound on the prominence of roaming in Chesnavich’s model. The bound highlights the intricacy of roaming as a type of dynamics on the verge between isomerisation and nonreactivity as it relies on generous access to the potential wells to allow reactions as well as a prominent area of high potential that aids sufficient transfer of energy between the degrees of freedom to prevent isomerisation.
In July 2018, three of the CHAMPS PDRA’s: Rafael Garcia Meseguer, Matthaios Katsanikas and Vladimir Krajnak, attended the workshop ‘Geometry of Chemical Reaction Dynamics in Gas and Condensed Phases’ at the Telluride Science Research Center in Telluride, CO, USA. The meeting focussed on theoretical aspects of chemical reaction dynamics and forms a platform for discussions about extensions of theory from lower to higher dimensional systems and from gas to condensed phases. Apart from meeting some of the leading researchers in the field and discussing ideas for future work, the PDRA’s also shared their contributions to the field. Rafael gave a talk on the applications of Lagrangian descriptors to finding dividing surfaces and invariant structures, Matthaios shared his findings on phase space transport in the Caldera model and Vladimir reported on the phase space mechanism behind Roaming in Chesnavich’s model and its evolution under parameter variations.
CHAMPS held a research day at Imperial College London on Friday 13th July 2018. This provided an opportunity for all of the investigators and PDRAs to discuss their research and obtain feedback from the entire CHAMPS team.
A copy of the schedule for the day can be found here: Research Day 13.07.18 Schedule
Professor Stephen Wiggins – Champs Principle Investigator
Professor Dmitry Shalashilin (left) Dr Dmitry Makhov (right)
Dr Matthaios Katsanikas – Champs PDRA
Dr Vladimir Krajnak – Champs PDRA
Professor Dmitry Shalashilin Co-Investigator (left) Professor Darryl Holm – Co-Investigator (right)
Dr Lars Bratholm – Champs PDRA
Professor Dmitry Shalashilin – Champs Co-Investigator
Professor Barry Carpenter – Champs Co-Investigator
Dr Shibabrat Naik – Champs PDRA
Dr Dmitry Makhov – Champs PDRA
Champs PDRAs – Makhov, Matthaios, Krajnak, Garcia-Meseguer, Naik, Bratholm (left to right)
Geometry of nonadiabatic quantum hydrodynamics
Michael. S. Foskett, Darryl. D. Holm, Cesare Tronci
(Submitted on 3 Jul 2018)
By using standard momentum maps from geometric mechanics, we collectivize the mean-field (MF) and exact factorization (EF) models of molecular quantum dynamics into two different quantum fluid models. After deriving the corresponding quantum fluid models, we regularize each of their Hamiltonians for finite ℏ by introducing spatial smoothing. The ℏ≠0 dynamics of the Lagrangian paths of the classical nuclear fluid flows for both MF and EF can be written and contrasted as finite dimensional canonical Hamiltonian systems for the evolution in phase space of singular solutions called Bohmions, in which each nucleus follows a Lagrangian path in configuration space. Comparison is also made with the variational dynamics of a new type of Bohmian trajectories, which arise from Hamilton’s principle with spatially smoothed quantum potential with finite ℏ.
The CHAMPS Leeds group just published in the Journal of Chemical Physics two papers related to CHAMPS Work Projects 4 and 5 on quantum dynamics and nonadiabatic dynamics. In the first paper “The effect of sampling techniques used in the multiconfigurational Ehrenfest method” by C.Symonds, J.Kattitzi and D.Shalashilin (https://aip.scitation.org/doi/10.1063/1.5020567) Spin-Boson model was used to assess the samplings of Canonical Coherent States basis sets in the phase space quantum mechanics. The paper validated the sampling techniques used in our simulations of ultrafast photochemical reactions. It has been demonstrated that the techniques really work and for the Spin-Boson model the calculations converge to the exact quantum result. The Figure below shows quantum wave functions in phase space.
In the second paper “Zombie states for description of structure and dynamics of multi-electron systems” by D.Shalashilin (https://aip.scitation.org/doi/10.1063/1.5023209) a new type of Fermionic Coherent State has been introduced, which potentially can be used in simulations of nonadiabatic dynamics in chemistry and photochemistry. Fermionic Coherent States are well known in mathematics. However, they require complicated algebra of Grassmann numbers not well suited for numerical simulations in computational chemistry. The paper introduces a coherent antisymmetrised superposition of “dead” and “alive” electronic states called Zombie State (ZS), which do not need Grassmann algebra. Instead it is replaced by a very simple sign-changing rule in the definition of creation and annihilation operators. Zombie States can be used as basis functions for quantum propagation just like Canonical Coherent States for distinguishable particles. As it is shown at the Figure in standard electronic structure and dynamics theories (left frame) some spin-orbitals are occupied by electrons and some are empty. In Zombie States (right frame) all orbitals are occupied, but some electrons are more “alive” than “dead” or more “dead” than “alive”. The term “Zombie” for simultaneously “dead” and “alive” electrons was proposed by Liz Clark, Bristol School of Mathematics manager, who is acknowledged in the paper!