A new method is described for studying collision dynamics in slow ion/atom–atom collisions. It is a generalization of the single-electron, two-center hidden crossings method to multielectron systems. This approach derives from the analytic properties of energy surfaces and wave functions of the adiabatic electronic Hamiltonian when the internuclear distance is extended into the complex plane. The collision dynamics in the adiabatic limit is determined by the topology of the unique multivalued electronic energy surface, particularly by its singular points, the square-root branch points. The surfaces described here have been studied using a complex version of the unrestricted Hartree–Fock and configuration interaction method with all single electron excitations, based on a bivariational principle. Although various inelastic processes can be calculated, the method is especially useful for the description of ionization. We have illustrated this through the calculation of cross sections for ionization of helium by proton and antiproton impact, as well as for the collision of two hydrogen atoms.

The method of hidden crossings is generalized to treat multielectron systems utilizing molecular Hartree-Fock and configuration interaction methods, extended into the plane of complex internuclear distance. Diabatic promotion of low lying states to the continuum in a two-electron, two-center system via a series of localized transitions is shown for the first time. Excellent agreement with experiments is found regarding single ionization in 50 eV to 1 keV H1H collisions.[S0031-9007 (96)01249-5] PACS numbers: 34.50.-s, 34.10.+x, 34.30.+hSlow atomic collisions involving several electrons play a substantial role in astrophysical, fusion, and technical plasma processes, in chemical reactions, and in collision spectroscopy. For example, knowledge of the ionization cross section in the collision of hydrogen atoms is critical for modeling the collapse of protostellar material as well as the divertor and edge regions of tokamak plasmas. Still, few reliable results are available. Experiments involving collisions of slow neutral particles are difficult, while theoretical calculations suffer because of the enormous number of molecular states needed for the reliable description of ionization.We introduce in this Letter an alternative method to the coupled channels approach and, for the fundamental case of ionization in H 1 H collisions, demonstrate its relative simplicity by readily identifying the main reaction channels, obtaining an accurate estimate of the cross section, and thus creating a new method for treating multielectron heavy particle collisions.The essential foundations and inspiration for the present work come from the "hidden crossing theory" [1]. In this approach the electronic quasimolecular Hamiltonian and its eigenfunctions are analytically continued into the plane of complex internuclear distance R. The resulting eigenvalue problem is no longer Hermitian and the complex eigenenergies of states of the same symmetry cross at isolated points R c , which have the structure of square-root branch points. The physical significance of such branch points lies in the fact that the radial matrix element between the states has a sharp local maximum at Re͕R c ͖, causing localization of the electronic transition in a slow atomic collision. The hidden crossing theory may be considered as a generalization of the multichannel Landau-Zener method to include those regions of localized nonadiabatic transitions not conventionally recognized as avoided crossings of the electronic quasimolecular terms. The main triumph of this theory when applied to two-center, one-electron systems has been the identification of a very localized series of branch points at small internuclear distances. This series has been called the S series and it represents a strong channel for ionization by almost diabatic promotion of low lying quasimolecular states into the continuum [1,2]. A main objective of this work is to confirm the existence of such a series in a two-center, multielectron system.The principal obstacle in applying hidden crossing theory...

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Many-body perturbation theory (MBPT) and couple-cluster doubles (CCD) calculation are reported for the formyl radical and for points on the potential energy surface corresponding to decomposition of the radical to hydrogen plus carbon monoxide. The predicted equilibrium structure (rCH=2.1 b, rco=2.245 b, and ϑ=124°) and dissociation energy (De=16.6 kcal mol−1) are in excellent agreement with experimental data. An analysis of the saddle-point region of the hypersurface provides a structure for the activated complex (rCH=3.35 b, rco=2.15 b, ϑ=115°) and predicts the critical energy (E0=18.5 kcal mol−1). Comparison of MBPT and CCD results for the dissociation energy and barrier height shows that equivalent results are obtained. A RRKM prediction of the decomposition rate coefficient for HCO→H+CO is also given.