Browsing by Subject "Potential energy surfaces"

Now showing 1 - 3 of 3
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    Adiabatic and Diabatic Energy Data for the Ground and First Excited Singlet States of CH₃NH₂
    (2020-05-18) Parker, Kelsey A; Truhlar, Donald G; truhlar@umn.edu; Truhlar, Donald G; Truhlar Group, Department of Chemistry, UMN-TC
    This data set includes adiabatic energies from XMS-CASPT2/6-31++G(d,p) calculations and diabatic energies and couplings calculated using the dipole-quadrupole diabatization method for the ground and first excited singlet states of methylamine (CH₃NH₂) at 1825 geometry points. This data was used to construct an analytical diabatic potential energy matrix.
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    Equilibrium Maps: Characterizing the complex and stochastic behavior of nanosystems subjected to proportional loading
    (2016-11) Pattamatta, A. S. L. Subrahmanyam
    Modeling systems subjected to proportional loading, such as a nanotube being pulled, tem- perature programmed desorption of chemical species on a surface etc, is of significant technological importance. A particularly challenging task when dealing with nanoscale systems is that the thermal fluctuations are on par with the applied stimulus owing to the small system size. Such fluctuations in conjunction with the extreme nonconvexity of the underlying potential energy hyper-surface (PES) means that there are mutually compet- ing pathways the system can take and the evolution of the system is stochastic. Often the evolution is affected by the rate at which the system is loaded. In the current work, we develop a novel solution strategy for simulating nanosystems sub- jected to proportional loading using branch following and bifurcation techniques. To this end, the concepts of the PES, the equilibrium points (ex. stable and transition states), and the transition networks connecting the stable states have been extended to the context of driven systems where the PES morphs in response to the external loading. We introduce the concept of the Equilibrium Map (EM), that is a distillation of the equilibrium and kinetic features of the evolving PES. The EM is then used to construct trajectories of the system for representative scenarios that span a spectrum of loading regimes and boundary conditions. We have developed efficient and highly scalable parallel codes to construct and handle the EM data. As a part of modeling the evolution of the system as a state to state dynamics, we have also addressed the issue of superbasins, arising due to clusters of stable states connected by low energy barriers relative to the barriers for transitions of the system to states out of the superbasin, in the context of an evolving PES. The proposed method is able to accelerate the system trapped in these superbasins and simulate the behavior of the system over long time scales. Finally we apply the EM method to a nanoslab under displacement controlled loading and show that the method qualitatively reproduces experimental observations on similar systems.
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    Multiscale Computational Analysis of Nitrogen and Oxygen Gas-Phase Thermochemistry in Hypersonic Flows
    (2016-02) Bender, Jason
    Understanding hypersonic aerodynamics is important for the design of next generation aerospace vehicles for space exploration, national security, and other applications. Ground-level experimental studies of hypersonic flows are difficult and expensive; thus, computational science plays a crucial role in this field. Computational fluid dynamics (CFD) simulations of extremely high-speed flows require models of chemical and thermal nonequilibrium processes, such as dissociation of diatomic molecules and vibrational energy relaxation. Current models are outdated and inadequate for advanced applications. We describe a multiscale computational study of gas-phase thermochemical processes in hypersonic flows, starting at the atomic scale and building systematically up to the continuum scale. The project was part of a larger effort centered on collaborations between aerospace scientists and computational chemists. We discuss the construction of potential energy surfaces for the N4, N2O2, and O4 systems, focusing especially on the multi-dimensional fitting problem. A new local fitting method named L-IMLS-G2 is presented and compared with a global fitting method. Then, we describe the theory of the quasiclassical trajectory (QCT) approach for modeling molecular collisions. We explain how we implemented the approach in a new parallel code for high-performance computing platforms. Results from billions of QCT simulations of high-energy N2 + N2, N2 + N, and N2 + O2 collisions are reported and analyzed. Reaction rate constants are calculated and sets of reactive trajectories are characterized at both thermal equilibrium and nonequilibrium conditions. The data shed light on fundamental mechanisms of dissociation and exchange reactions – and their coupling to internal energy transfer processes – in thermal environments typical of hypersonic flows. We discuss how the outcomes of this investigation and other related studies lay a rigorous foundation for new macroscopic models for hypersonic CFD. This research was supported by the Department of Energy Computational Science Graduate Fellowship and by the Air Force Office of Scientific Research Multidisciplinary University Research Initiative.