Effective Fragment Potentials for Microsolvated Excited and Anionic States.

The effective fragment potential (EFP) approach is a sophisticated hybrid approach that allows the inclusion of solvation effects when describing properties and reactivity in the condensed phase, without using empirical parameters. This work examines the performance of the EFP method when describing microsolvation in electronically excited states of neutrals and anions. The examples selected include both localized valence states, as well as diffuse nonvalence states, which represent greater challenges to conventional electronic structure methods. The equation-of-motion coupled cluster with singles and doubles (EOM-XX-CCSD) methodology has been used to provide the quantum chemical description of both the full microsolvated clusters, and the chromophoric moiety in mixed quantum/EFP calculations. We find that, when averaging over multiple configurations of microsolvated clusters, the differences between QM/EFP and full quantum results are minimal, although individual configurations often have larger errors. As expected, diffuse states have somewhat larger errors, although not significantly so. The close proximity of states leading to mixing can make QM/EFP less accurate because a change of ordering of states can occur. Other properties, such as photoelectron images and lifetimes of metastable states, are very well described for the monohydrated clusters investigated.

Spectroscopy and Theoretical Modeling of Tetracene Anion Resonances.

The positions and widths of the optically allowed electronic states of the tetracene radical anion located above the detachment threshold energy (i.e anion resonances) are mapped out using total photodetachment yield spectroscopy of cryogenically cooled ions. The presence of these states is detected via the sharp increase in the photodetachment yield compared to that of the monotonic nonresonant direct photodetachment background. The resolution of the resulting spectrum is limited by the ∼5 cm-1 line width of the tunable laser and thus provides a stringent benchmark for computations of the energies and autodetachment lifetimes of these resonance states. The experimental results are compared to high-level electronic structure computations and line width modeling using the orbital stabilization method. These theoretical results are found to be in near quantitative agreement with the experimental data, highlighting their capability to accurately describe the energies and lifetimes of anion resonances for relatively large molecules.

Projected Complex Absorbing Potential Multireference Configuration Interaction Approach for Shape and Feshbach Resonances.

Anion resonances are formed as metastable intermediates in low-energy electron-induced reactions. Due to the finite lifetimes of resonances, applying standard Hermitian formalism for their characterization presents a vexing problem for computational chemists. Numerous modifications to conventional quantum chemical methods have enabled satisfactory characterization of resonances, but specific issues remain, especially in describing two-particle one-hole (2p-1h) resonances. An accurate description of these resonances and their coupling with single-particle resonances requires a multireference approach. We propose a projected complex absorbing potential (CAP) implementation within the multireference configuration interaction (MRCI) framework to characterize single-particle and 2p-1h resonances. As a first application, we use the projected-CAP-MRCI approach to characterize and benchmark the 2Πg shape resonance in N2-. We test its performance as a function of the size of the subspace and other parameters, and we compute the complex potential energy surface of the 2Πg shape resonance to show that a smooth curve is obtained. One key benefit of MRCI is that it can describe Feshbach resonances (most common examples of 2p-1h resonances) at the same footing as shape resonances. Therefore, it is uniquely positioned to describe mixing between the different channels. To test these additional capabilities, we compute Feshbach resonances in H2O- and anions of dicyanoethylene isomers. We find that CAP-MRCI can efficiently capture the mixing between the Feshbach and shape resonances in dicyanoethylene isomers, which has significant consequences for their lifetimes.

A Unique QP Partitioning and Siegert Width Using Real-Valued Continuum-Remover Potential.

A simple, practical quantum chemical procedure is presented for computing the energy position and the decay width of autoionization resonances. It combines the L2-stabilized resonance wave function obtained using the real-valued continuum-remover (CR) potential [Y. Sajeev Chem. Phys. Lett. 2013, 587, 105-112] and the Feshbach projection operator (FPO) partitioning technique. Unlike the conventional FPO partitioning of the total wave function into its resonant Q space and background P space components, an explicit partitioning of the total wave function into its interaction region and noninteraction region components is obtained with the help of real-valued continuum-remover potential. The molecular system is initially confined inside a CR potential which removes the electronic continuum of the molecular system in which its resonance state is embedded and, thus, unravels the Q space component of the resonance wave function as a bound, localized eigenstate of the confined system. The eigenfunctions of the molecular Hamiltonian represented in the {1-Q} space constitute a complementary, orthogonal P space. A unique QP partition is obtained when the level-shift of the Q space function due to its coupling with the P space is zero, and the resonance width is computed using these unique partitioned spaces. This new procedure, which we refer to as CR-FPO formalism, is formally very simple and straightforward to implement, yet its applications to the resonance state of a model Hamiltonian and to the doubly excited resonance states of atomic and molecular systems at the full-CI level are very accurate as compared to the alternative, very precise L2 methods. In addition, the CR-FPO formalism is implemented in the multireference configuration interaction (MRCI) method, and uses it for calculating the energy position and the autionization decay width of 2Πg shape resonance in N2-.

A Simple and Efficient Method for Simulating the Electronic Absorption Spectra of Criegee Intermediates: Benchmarking on CH2OO and CH3CHOO.

Criegee intermediates (CIs) play a vital role in the atmosphere-known most prominently for enhancing the oxidizing capacity of the troposphere. Knowledge of their electronic absorption spectra is of vital importance for two reasons: (1) to aid experimentalists in detecting CIs and (2) in deciding if their removal is affected by solar photolysis. In this article we report a simple and efficient method based on the nuclear ensemble method that may be effectively used to compute the electronic absorption spectra of Criegee intermediates without the need for extensive computation of preparing the initial configurations of the starting geometry. We use this method to benchmark several excited-state electronic structure methods and their efficacy in reproducing the electronic absorption spectra of two well-known cases of CI: CH2OO and CH3CHOO. The success and computational feasibility of the methodology are crucial for its applicability to CIs of increasing molecular complexity, which have no known experimentally measured electronic absorption spectra, allowing a guide for experimentalists. Application of the methodology to more complex CIs (e.g., those with extended conjugation or those derived from endocyclic alkenes) will also reveal if solar photolysis becomes a competitive removal process when compared to unimolecular decay or bimolecular chemistry.

Description of Two-Particle One-Hole Electronic Resonances Using Orbital Stabilization Methods.

Two-particle one-hole (2p-1h) resonances are elusive to accurate characterization, their decay to the neutral state being a two-electron process. Although in limited cases, single reference methods can be used, a proper description of a 2p-1h resonant state entails a multiconfigurational treatment of the reference wavefunction. In this work, we test the performance of the orbital stabilization method to characterize the 2p-1h resonances found in water and benzene. We employ a set of two multireference approaches, namely, the restricted active space self-consistent field and the multireference configuration interaction, as well as the single reference method equation of motion for electron attachment coupled-cluster with singles and doubles, in the case of benzene. We further explore the resonant channel mixing in benzene between the B2g shape resonance and 2p-1h resonance, a phenomenon which has been explored quite often in experimental studies.

Comparative study of methodologies for calculating metastable states of small to medium-sized molecules.

We present a benchmarking study on the performance of two methods at the forefront of studying electronic metastable states of molecules: the orbital stabilization method and the method of complex absorbing potential augmented Hamiltonians. The performance of the two methods is compared for the calculation of shape resonances in small to medium-sized molecules (up to 15 atoms) at the equation of motion coupled cluster with singles and doubles for the electron attachment level of methodology using even-tempered Gaussian basis sets. The theoretical positions and widths of shape resonances obtained from both methods are compared to the experimentally determined electron affinities and lifetimes. The challenges that accompany the theoretical estimation of resonance positions and widths for medium to large-sized systems with an increase in basis set size are also discussed.