By adding a negative imaginary potential of variable strength eta to the Hamiltonian, the resonance state of a system can be found as complex energy stabilized points in the eta-trajectories of the eigenvalues. One problem that arises in practical calculations is the appearance of nonphysical complex energy stabilized points. A new method for separating the physical from the nonphysical complex energy stabilized points is proposed. The method is best illustrated with strongly correlated two-electron systems.

In low-energy electron impact on neutral molecules, the free electron and the molecule often form an intermediate metastable electron-molecule compound. [1][2][3] This compound may either release the excess electron by autodetachment (AD) [1] or fragment by a reaction called dissociative electron attachment [DEA, Eq. (1)]: [1][2][3] e À þ AB ! ðABÞA wide variety of chemical transformations initiated by electron impact can be attributed to DEA. [1][2][3] In particular, it is well known that in organic molecules DEA efficiently leads to bond dissociation, producing radical and radical-anion molecular fragments. In general, in all examples studied and understood theoretically, DEA leads to an anionic fragment and a neutral fragment. [1][2][3] Recently, mass spectroscopic measurements monitoring DEA of fructose have shown that several neutral fragments may appear in addition to the anionic fragment.[4] The underlying mechanisms have not been clarified yet, but it may be suspected from the detected products that the two fragments initially formed by DEA as in Equation (1) have further fragmented by several stepwise reactions. In any case, electron-molecule reactions are seen to be chemically rich and are of chemical interest.Herein we report on a new elementary reaction (i.e., single-step reaction) mechanism of an electron and a molecule in a metastable compound which we call (two or more) bond breaking by a catalytic electron (BBCE). Unlike in DEA, the formed compound anion dissociates into nonradical neutral molecular subunits and a free electron, which plays the role of a catalyst [Eq. (2)]:where we stress that at least two bonds (not a double bond) break in the elementary reaction path. Notice that the electron is freed in the course of the elementary reaction, that is, the electron is not attached to any of the chemical products of the elementary reaction and, thus, we refer to this mechanism as an electron-catalyzed mechanism. This elementary reaction involves both bond breaking and detachment of the electron. The key differences between BBCE and DEA are as follows: 1. More than one s bond (two-center bond) is broken in the elementary BBCE reaction. 2. The products formed in the BBCE are neutral. 3. The electron is released in the course of the elementary BBCE reaction.Below, we illustrate this mechanism more precisely by investigating electron impact on the quadricyclanone (QDCO) molecule. As will become clearer, the low-energy electron impact on QDCO can be viewed as proceeding via a compound negative ion metastable state. We have acquired considerable experience in the ab initio computation of energy and lifetime of metastable anions using non-Hermitian quantum chemical methods. [5][6][7] Recently, the introduction of a so-called continuum remover complex absorbing potential [8] and its implementation in Greens function methods [9] have made non-Hermitian quantum chemical methods applicable to larger systems. The efficient identification of the metastable states and the correct scaling of the electronic ener...

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The reflection-free complex absorbing potential (RF-CAP) method has been already applied to the study of the autoionization resonance of helium [Sajeev et al., Chem. Phys. 329, 307 (2006)]. The present work introduces a systematic way for implementing RF-CAP for the electronic structure calculations using Gaussian basis sets for molecules. As a test case study we applied the RF-CAP method to the lowest (1)Sigma(g) (+) and (1)Sigma(u) (+) Feshbach-type autoionization resonances of hydrogen molecule. Since thin RF-CAP absorbs fast electrons much better than the slow ones, a weak dc field has been added to the RF-CAP in the peripheral region of the molecule.

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We have formulated and applied an analytic continuation method for the recently formulated correlated independent particle potential [A. Beste and R. J. Bartlett J. Chem. Phys. 120, 8395 (2004)] derived from Fock space multireference coupled cluster theory. The technique developed is an advanced ab initio tool for calculating the properties of resonances in the low-energy electron-molecule collision problem. The proposed method quantitatively describes elastic electron-molecule scattering below the first electronically inelastic threshold. A complex absorbing potential is utilized to define the analytic continuation for the potential. A separate treatment of electron correlation and relaxation effects for the projectile-target system and the analytic continuation using the complex absorbing potential is possible, when an approximated form of the correlated complex independent particle potential is used. The method, which is referred to as complex absorbing potential-based correlated independent particle (CAP-CIP), is tested by application to the well-known (2)Pi(g) shape resonance of e-N(2) and the (2)B(2g) shape resonance of e-C(2)H(4) (ethylene) with highly satisfactory results.

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Inclusion of selected higher excitations involving active orbitals in the state-specific multireference coupledcluster theory Analytic evaluation of the nonadiabatic coupling vector between excited states using equation-of-motion coupledcluster theory Fock space multireference coupled cluster calculations based on an underlying bivariational self-consistent field on Auger and shape resonances Resonances in S N 2 reactions: Two-mode quantum calculations for Cl − + CH 3 Br on a coupled-cluster potential energy surfaceThe technique of Fock space multireference coupled-cluster ͑FSMRCC͒ is applied for the first time to the correlated calculation of the energy and width of a shape resonance in an electron-molecule collision. The procedure is based upon combining a complex absorbing potential with FSMRCC theory. Accurate resonance parameters are obtained by solving a small non-Hermitian eigenvalue problem. The potential-energy curve of the 2 ⌸ g state of N 2 − is calculated using the FSMRCC and multireference configuration-interaction ͑MRCI͒ level of theories. Comparison with the single-determinant Hartree-Fock theory indicates that correlation effects are important in determining the behavior of the resonance state.