Potential energy surface overview

Chapter 3, Geometry Optimizations, describes how to locate equilibrium structures of molecules, or, more technically, stationary points on the potential energy surface. It includes an overview of the various commonly used optimization techniques and a consideration of optimizing transition strucmres as well as minimizations. 

We present an overview of our research program on HF-HF collisions, including potential energy surfaces and dynamics calculations, with special emphasis on the sensitivity of the dynamics results to the choice of surface. 

An overview of the time-dependent wavepacket propagation approach for four-atom reactions together with the construction of ab initio potential energy surfaces sufficiently accurate for quantum dynamics calculations has been presented. Today, we are able to perform the full-dimensional (six degrees-of-freedom) quantum dynamics calculations for four-atom reactions. With the most accurate YZCL2 surface for the benchmark four-atom reaction H2 + OH <-> H+H2O and its isotopic analogs, we were able to show the following ... 

Schlegel, H. B. 2003. Exploring Potential Energy Surfaces for Chemical Reactions An Overview of Some Practical Methods , J. Comput. Chem. 124, 1514. 

The purpose of this chapter is a detailed comparison of these systems and the elucidation of the transition from regular to irregular dynamics or from mode-specific to statistical behavior. The main focus will be the intimate relationship between the multidimensional PES on one hand and observables like dissociation rate and final-state distributions on the other. Another important question is the rigorous test of statistical methods for these systems, in comparison to quantum mechanical as well as classical calculations. The chapter is organized in the following way The three potential-energy surfaces and the quantum mechanical dynamics calculations are briefly described in Sections II and III, respectively. The results for HCO, DCO, HNO, and H02 are discussed in Sections IV-VII, and the overview ends with a short summary in Section VIII. 

D. A. Micha. Potential Energy Surfaces and Dynamics Calculations, chapter Overview of non-reactive scattering, page 685. Plenum Press, New York, 1981. 

Schlegel, H.B., Exploring potential energy surfaces for chemical reactions an overview of some practical methods, J. Computational Chem., 24, 1514—1527, 2003. 

A general overview of the different potential energy surfaces relevant to the reactions involving an oxygen atom and the hydrogen molecule has been sketched in the work of Durand and Chapuisat [51]. 

To get an overview of the relevant potential energy surfaces, several stationary points of each system were calculated with the 6-311+G basis set and the B3LYP functional using the Gaussian package of programs [5]. For the structural identification of the expected species it was also necessary to obtain the calculated vibrational spectra. 

The paper is organized as follows. In section 2, we provide the relevant background information about the MMCC formalism and overview the CR-CC methods employed in this study. In section 3, we examine the performance of various CC and CR-CC methods in calculations for the three cuts of the water potential energy surface described above and compare the results with those obtained with MRCI(Q) and the ES potential function. Finally, in section 4, we provide the concluding remarks. 

Strategies for walking on potential energy surfaces are overviewed by... 

The reactions H + H2 and F + H2 (and their isotopic variants) have been the benchmark systems in the field of chemical reaction dynamics. For them, fully converged three-dimensional (3D) quantum scattering calculations of state-to-state differential cross sections (DCS) have been performed and accurate comparisons with very detailed experimental observables carried out [3-9]. To date, only for one other neutral three-atom system have the exact (i.e., fully converged) 3D quantum scattering calculations of state-to-state DCS on a reliable ab initio potential energy surface (PES) been carried out, namely, for the prototypical reaction Cl + H2 [10], a system chemical kineticists have been interested in since the time of Max Bodenstein (for a historical overview, see the paper by Truhlar in this volume). However, in contrast to H + H2 and F + H2, no experimental dynamical information is available on Cl + H2. Here we highlight the results of the first dynamical investigation of the Cl + H2 and Cl + D2 reactions by the crossed molecular beam... 

Section II gives a simple overview of our approach for calculating the van der Waals spectra. Section III gives brief details of the potential energy surfaces used. In Section IV we present our predicted spectra and compare with experimental data when available. The comparison of the different spectra we have calculated also illustrates many interesting points about how van der Waals spectra vary with the complexity of the system and the structure and anisotropy of the potential energy surface. Conclusions are in Section V. 

Chemical dynamics is the link between the potential energy surface (PES) (or surfaces) and an observable chemical phenomena. In principle the PES comes from an ab initio quantum chemistry calculation (within the Born-Oppenheimer approximation) though in practice it is often constructed by some more approximate model, e.g., semiempirical quantum chemistry or totally empirical force field models. First a brief overview of the present state of the methodology and scope of applications in this area is given. We will concentrate on chemical dynamics in the gas phase, though much of the methodology of this field has carried over to the study of dynamical processes in condensed phases, gas-surface collision processes, and also dynamics in biomolecular systems. 

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