Molecular surface scattering computations

In Chapter VI, Ohm and Deumens present their electron nuclear dynamics (END) time-dependent, nonadiabatic, theoretical, and computational approach to the study of molecular processes. This approach stresses the analysis of such processes in terms of dynamical, time-evolving states rather than stationary molecular states. Thus, rovibrational and scattering states are reduced to less prominent roles as is the case in most modem wavepacket treatments of molecular reaction dynamics. Unlike most theoretical methods, END also relegates electronic stationary states, potential energy surfaces, adiabatic and diabatic descriptions, and nonadiabatic coupling terms to the background in favor of a dynamic, time-evolving description of all electrons. 

Quantum-chemical cluster models, 34 131-202 computer programs, 34 134 methods, 34 135-138 for chemisorption, 34 135 the local approach, 34 132 molecular orbital methods, 34 135 for surface structures, 34 135 valence bond method, 34 135 Quantum chemistry, heat of chemisorption determination, 37 151-154 Quantum conversion, in chloroplasts, 14 1 Quantum mechanical simulations bond activation, 42 2, 84—107 Quasi-elastic neutron scattering benzene... 

The structure of the adsorbed ion coordination shell is determined by the competition between the water-ion and the metal-ion interactions, and by the constraints imposed on the water by the metal surface. This structure can be characterized by water-ion radial distribution functions and water-ion orientational probability distribution functions. Much is known about this structure from X-ray and neutron scattering measurements performed in bulk solutions, and these are generally in agreement with computer simulations. The goal of molecular dynamics simulations of ions at the metal/water interface has been to examine to what degree the structure of the ion solvation shell is modified at the interface. 

The main purpose of the method is to define molecular shapes through isodensity surfaces. Tests on a number of small molecules show that this aim is achieved with a great efficiency in computer time. Discrepancies between MEDLA densities and theoretical distributions, averaged over the grid points, are typically below 10% of the total density. While this does not correspond to an adequate accuracy for an X-ray scattering model, the results do provide important information on the shapes of macromolecules. 

Gerber, R.B., Kosloff, R., and Berman, M. (1986). Time-dependent wavepacket calculations of molecular scattering from surfaces, Computer Physics Reports 5, 59-114. 

Fig. 8. Voltage drop Vi, /, z) in the scattering region is presented for bias of 0.5 V applied to the left Au electrode. The electrode separation is of 11.7 A, and the C60 sits in the middle of the tunnel junction. Here, we define the voltage drop in the scattering region as Vj,(r) = Vn(r Vj,) — Vjj(r Vj, = 0). The presented Vj,(y, z) is an average over the horizontal planes, i.e., f dxV, (x,y,z)/(xt — xt>), where xt, and xt are the coordinates of the bottom and top of the computation box in the x direction. The dashed vertical lines highlight the C60 location in the molecular junction. We project the positions of the left and right electrodes and the molecule onto the surface Vf,(y, z). The corresponding edges are shown by the bold solid lines.

For 4-atom systems such as formaldehyde the photodissociation dynamics have yet to be established. A fully quantal description of the photodissociation is still very expensive computationally. So far 6-dimensional wave packet studies have been applied only to one electronic surface, for example for scattering of a molecule or atoms on surfaces or collisions of molecules. Currently, for molecular systems with more than four atoms, a reduced dimensionality model must be used. This means that certain degrees of freedom are fixed throughout the dynamical simulation. 

The first block, INTERACTION, is devoted to the calculation of electronic energies determining the potential energy surface (PES) on which the nuclear morion takes place. The second l)lock, DYNAMICS, is devoted to the integration of the scattering equations to determine the outcome of the molecular process. The third block, OBSER WBLES. is devoted to the reconstruction of the ol)serr al)le properties of the beam from the calculated dynamical quantities. All these blocks reejuiro not only different skills and expertise but also specialized computer software and hardware. 

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