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We introduce a protocol to represent quantum states as a linear superposition of model electronic diabatic basis states coupled in an external (static) electric field. By considering an entire family of these models, we uncover trends in reaction-path geometry and the topology of potential-energy surfaces, including all those that can be realized in a two-dimensional configurational space. Our approach can be used as a tool to model the key parameters (e.g., diabatic basis states, external field intensity) that yield desired geometrical characteristics in an actual potential energy surface. In this work, external agents such as laser fields, or a group of neighboring charges, are regarded as essential requirements to prompt, or trigger, the occurrence of a chemical process. In these cases, reaction path geometry can be modulated externally so as to yield processes that would appear to occur far from gas-phase geometries. This phenomenology is intrinsically nonadiabatic. Our present approach accounts for the possibility of such features, i.e., the occurrence of quantum states whose electronic structures resemble products, while at geometries that are more similar to those of reactants. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Active site properties in some proteins can be affected by conformational fluctuations of neighbor residues, even when the latter are not involved directly in the binding process. A local environment thus appears to alter the relevant potential energy surface and its reaction paths. Here, some aspects of this phenomenon are simulated within a generalized electronic diabatic (GED) scheme to study the geometry and structural similarity for a class of two-dimensional (2D) energy surfaces. The electronic quantum state is a linear superposition of diabatic basis functions, each of which is taken to represent a single (pure) electronic state for the isolated material system. Here, we describe a model reaction of isomerization by shifts in amplitudes for three diabatic species (reactant, product, and an open-shell transition state) coupled in an external field. The “effective” 2D energy surface in the field is characterized in terms of critical points, and the amplitudes along the main reaction paths. A new feature is the introduction of a phase diagram where all possible potential-energy-surface topologies (consistent with three-state systems in two linear coordinates) are matched with actual model parameters. By varying the coupling strengths between diabatic states, we classify regions of this phase diagram in terms of electronic and structural similarities; some regions comprise models whose reaction paths have geometries that belong to the catchment region of the reactant, yet are electronically akin to the diabatic transition state or product. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Reaction paths are studied with the help of diabatic potential-energy surfaces coupled in a generic external field. We show that all putative geometrical and topological features of two-dimensional (2D) potential-energy surfaces for an isomerization can be generated with a model consisting of strictly diabatic harmonic 2D wells coupled in a static (yet uniform) external field. For a large class of three-state models, we provide a phase diagram of possible topologies in the relevant parameter space for three-state models. By following the shift in diabatic electronic amplitudes (denoted by {|ck|2}) in coherent quantum states produced in the field, we assess whether the models within each phase can produce reaction paths visiting regions of configurational space that are structurally similar to the diabatic transition state. [ABSTRACT FROM AUTHOR]