Shape resonance induced electron attachment to cytosine: The effect of aqueous media.

We have investigated the impact of microsolvation on shape-type resonance states of nucleobases, taking cytosine as a case study. To characterize the resonance position and decay width of the metastable states, we employed the newly developed DLPNO-based EA-EOM-CCSD method in conjunction with the resonance via Padé (RVP) method. Our calculations show that the presence of water molecules causes a redshift in the resonance position and an increase in the lifetime for the three lowest-lying resonance states of cytosine. Furthermore, there are some indications that the lowest resonance state in isolated cytosine may get converted to a bound state in the presence of an aqueous environment. The obtained results are extremely sensitive to the basis set used for the calculations.

Complex energies and transition dipoles for shape-type resonances of uracil anion from stabilization curves via Padé.

Absorption of slow moving electrons by neutral ground state nucleobases has been known to produce resonance metastable states. There are indications that such metastable states may play a key role in DNA/RNA damage. Therefore, herein, we present an ab initio non-Hermitian investigation of the resonance positions and decay rates for the low lying shape-type states of the uracil anion. In addition, we calculate the complex transition dipoles between these resonance states. We employ the resonance via Padé (RVP) method to calculate these complex properties from real stabilization curves by analytical dilation into the complex plane. This method has already been successfully applied to many small molecular systems, and herein, we present the first application of RVP to a medium-sized system. The presented resonance energies are optimized with respect to the size of the basis set and compared with previous theoretical studies and experimental findings. Complex transition dipoles between the shape-type resonances are computed using the optimal basis set. The ability to calculate ab initio energies and lifetimes of biologically relevant systems paves the way for studying reactions of such systems in which autoionization takes place, while the ability to also calculate their complex transition dipoles opens the door for studying photo-induced dynamics of such biological molecules.

The Clusterization Technique: A Systematic Search for the Resonance Energies Obtained via Padé.

Atomic and molecular resonances play an important role in many physical processes, hence developing theoretical tools to properly calculate these states is required. Recently, we introduced a method for calculating the electronic resonance complex energies from stabilization graphs via analytical continuation, specifically, using the Padé approximant. This method was shown to be efficient, for example, in interpreting the results of cold molecular collisions. However, we observed that the complex energies obtained by Padé depend on the selected set of input points from the stabilization graph. In addition, unphysical solutions (noise) may appear and need to be eliminated. Therefore, applying the method to systems in which the resonance values are unavailable is difficult. The excited Li-He* Feshbach states, for which autoionization was recently observed, present such a challenge. Herein, we introduce a statistical approach to single out the resonance energy from the false solutions by identifying it as a cluster of Padé solutions. This clusterization technique was applied to study several electronic resonance states, for which we obtained excellent agreement with available data (exact or other theoretical solutions and an experimental result). Following this, the technique successfully identified the most likely Li-He* Feshbach resonance energy. Moreover, we concluded that large input sets generate much noise while restricting the number of points facilitates clusterization, which makes this approach more attractive, since input points are obtained from computationally demanding electronic-structure calculations. Overall, the use of analytical continuation via Padé, along with the statistical technique presented herein, offers an efficient approach to calculate resonances.

On the calculation of resonances by analytic continuation of eigenvalues from the stabilization graph.

Resonances play a major role in a large variety of fields in physics and chemistry. Accordingly, there is a growing interest in methods designed to calculate them. Recently, Landau et al. proposed a new approach to analytically dilate a single eigenvalue from the stabilization graph into the complex plane. This approach, termed Resonances Via Padé (RVP), utilizes the Padé approximant and is based on a unique analysis of the stabilization graph. Yet, analytic continuation of eigenvalues from the stabilization graph into the complex plane is not a new idea. In 1975, Jordan suggested an analytic continuation method based on the branch point structure of the stabilization graph. The method was later modified by McCurdy and McNutt, and it is still being used today. We refer to this method as the Truncated Characteristic Polynomial (TCP) method. In this manuscript, we perform an in-depth comparison between the RVP and the TCP methods. We demonstrate that while both methods are important and complementary, the advantage of one method over the other is problem-dependent. Illustrative examples are provided in the manuscript.

Atomic and Molecular Complex Resonances from Real Eigenvalues Using Standard (Hermitian) Electronic Structure Calculations.

Complex eigenvalues, resonances, play an important role in a large variety of fields in physics and chemistry. For example, in cold molecular collision experiments and electron scattering experiments, autoionizing and predissociative metastable resonances are generated. However, the computation of complex resonance requires modifications of standard electronic structure codes and methods, which are not always straightforward, in addition, application of complex codes requires more computational efforts. Here we show how resonance eigenvalues, positions and widths, can be calculated using the standard, widely used, electronic-structure packages. Our method enables the calculations of the complex resonance eigenvalues by using analytical continuation procedures (such as Padé). The key point in our approach is the existence of narrow analytical passages from the real axis to the complex energy plane. In fact, the existence of these analytical passages relies on using finite basis sets. These passages become narrower as the basis set becomes more complete, whereas in the exact limit, these passages to the complex plane are closed. As illustrative numerical examples we calculated the autoionization Feshbach resonances of helium, hydrogen anion, and hydrogen molecule. We show that our results are in an excellent agreement with the results obtained by other theoretical methods and with available experimental results.

Characteristic footprints of an exceptional point in the dynamics of Li dimer under a laser field.

Non-hermitian quantum mechanics is a formalism that excels in describing time-dependent states such as resonances. As one, it opens up a window to explore new and undiscovered phenomena. Under this formalism coalescence of two eigenstates and a deficient spectrum are a possible situation. These situations are unique and can occur solely in specific conditions known as Exceptional Points (EPs). An EP holds unique characteristics. One of which is a switch-like behavior: upon adiabatically changing the conditions in a closed loop around the EP, the population of one resonance can be transferred completely to another resonance. The phenomenon was not experimentally observed in an atomic or molecular system so far, although experiments involving nonlinear PT symmetry optics and microwave cavities have already indicated its existence. In this work, we demonstrate and confirm that the switch-like behavior exists in the spectrum of a lithium dimer taking into account both the rotations and the vibrations of the system. Moreover, a footprint of the EP is also shown to exist in the photo-association process of the lithium dimer. In this process, the EP's resonances serve as the mean to associate two free lithium atoms into a dimer. Based on this, we suggest a corresponding experiment to demonstrate for the first time the EP phenomenon in a molecular system.

Quantum Effects in Cold Molecular Collisions from Spatial Polarization of Electronic Wave Function.

The quantum phenomena of electronic and nuclear resonances are associated with structures in measured cross sections. Such structures were recently reported in a cold chemistry experiment of ground-state hydrogen isotopologues (H2/HD) colliding with helium atoms in the excited triplet P-state (He(23P)) [Shagam et al. Nature Chem. 2015, 7, 921], but a theoretical explanation of their appearance was not given. This work presents a quantum explanation and simulation of this experiment, which are strictly based on ab initio calculations. We incorporate complex potential energy surfaces into adiabatic variational theory, thereby reducing the multidimensional scattering process to a series of uncoupled 1D scattering "gedanken experiments". Our theoretical result, which is in remarkable agreement with the experimental data, manifests that the structures in the observed reaction rate coefficient are due to the spatial arrangement of the excited He p-orbitals with respect to the interaction axis, consequently changing the system from a normal two-rotor model to a three-rotor one. This theoretical scheme can be applied to explain and predict cross sections or reaction rate coefficients for any resonance-related phenomenon.

Polyatomic ab Initio Complex Potential Energy Surfaces: Illustration of Ultracold Collisions.

Resonances are metastable states that decay after a finite period of time. These states play a role in many physical processes. For example, in recent cold collision experiments, autoionization from a resonance state was observed. Complementing such observations with theory provides insight into the reaction dynamics under study. Theoretical investigation of autoionization processes is enabled via complex potential energy surfaces (CPESs), where the real and imaginary parts, respectively, provide the energy and decay rate of the system. Unfortunately, calculation of ab initio polyatomic CPESs are cumbersome; hence, they are not in abundance. Here, we present an ab initio polyatomic CPES utilizing a recently developed approach, which makes such calculations feasible. This CPES helps interpret the autoionization process observed in the He(23S) + H2 collision. From the behavior of the calculated CPES we can conclusively determine the nature of the autoionization process. Moreover, this CPES was used to generate reaction rates for the collision of He with ortho- and para-H2. These reaction rates are obtained from first principles. The results show a remarkable agreement with the cold collision experimental measurements, which demonstrates the robustness of our method. Hereby, we provide a computational tool for designing and interpreting new types of experiments that involve resonance states, e.g., in nucleobase damages (DNA or RNA) or in interatomic (intermolecular) Coulombic decay.