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The anions C$_7$N$^-$ and C$_{10}$H$^-$ are the two longest of the linear (C,N)-bearing and (C,H)-bearing chains which have so far been detected in the Interstellar Medium. In order to glean information on their collision-induced rotational state-changing processes, we analyse the general features of new ab initio potentials describing the interaction of both linear anions with H$_2$, one of the most abundant partners in their ISM environment. We employ an artificial neural network fit of the reduced-dimensionality potential energy surface for C$_7$N$^-$...H$_2$ interaction and discuss in detail the spatial features in terms of multipolar radial coefficients. For the C$_{10}$H$^-$...H$_2$ interaction we use the initial grid of two dimensional raw points to generate by quadrature the Legendre expansion directly, further including the long-range terms as discussed in the main text. Quantum scattering calculations are employed to obtain rotationally inelastic cross sections, for collision energies in the range of 10$^{-4}$ to 400 cm$^{-1}$. From them we generate the corresponding inelastic rate coefficients as a function of temperature covering the range from 10 to 50 K. The results for the rate coefficients for the longest cyanopolyyne are compared with the earlier results obtained for the smaller terms of the same series, also in collision with H$_2$. We obtain that the inelastic rate coefficients for the long linear anions are all fairly large compared with the earlier systems. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are illustrated in our conclusions.

We report new results from quantum calculations of energy-transfer processes taking place in interstellar environments and involving two newly observed molecular species: C$_5$N$^-$ and C$_7$N$^-$ in collision with He atoms and the p-H$_2$ molecules. These species are part of the anionic molecular chains labeled as cyanopolyynes which have been observed over the years in molecule-rich Circumstellar Envelopes and in molecular clouds. In the present work, we first carry out new $ab$ $initio$ calculations for the C$_7$N$^-$ interaction potential with He atom and then obtain state-to-state rotationally inelastic cross sections and rate coefficients involving the same transitions which have been observed experimentally by emission in the interstellar medium (ISM) from both of these linear species. For the C$_5$N$^-$/He system we extend the calculations already published in our earlier work (see reference below) to compare more directly the two molecular anions. We extend further the quantum calculations by also computing in this work collision rate coefficients for the hydrogen molecule interacting with C5N$^-$, using our previously computed interaction potential. Additionally, we obtain the same rate coefficients for the C$_7$N$^-$/H$_2$ system by using a scaling procedure that makes use of the new C$_7$N$^-$/He rate coefficients, as discussed in detail in the present paper. Their significance in affecting internal state populations in ISM environments where the title anions have been found is analyzed by using the concept of critical density indicators. Finally, similarities and differences between such species and the comparative efficiency of their collision rate coefficients are discussed. These new calculations suggest that, at least for the case of these longer chains, the rotational populations could reach local thermal equilibrium conditions within their observational environments.

We present in this paper the main structural features and enthalpy details for the energy profiles of the title reactions, both for the exothermic (forward) path to NH$_{3}$ formation and for the endothermic (reverse) reaction to NH$_{2}^{-}$ formation. Both systems have relevance for the nitrogen chemistry in the interstellar medium (ISM). They are also helpful to document the possible role of H$^{-}$ in molecular clouds at temperatures well below room temperature. The structural calculations are carried out using ab initio methods and are further employed to obtain the reaction rates down to the interstellar temperatures detected in earlier experiments. The reaction rates are obtained from the computed Minimum Energy Path (MEP) using the Variational Transition State Theory (VTST) approach. The results indicate very good accord with the experiments at room temperature, while the measured low temperature data down to 8 K are well described once we analyse in detail the physics of the reactions and modify accordingly the VTST approach. This is done by employing a T-dependent scaling, from room temperature conditions down to the lower ISM temperatures, which acknowledges the non-canonical behavior of the fast, barrierless exothermic reaction. This feature was also suggested in the earlier work discussed below in our main text. The physical reasons for the experimental behavior, and the need for improving on the VTST method when used away from room temperatures, are discussed in detail.

Using the ab initio computed Potential Energy Surface (PES) for the electronic interaction of the MgH$^+$ ($^1\Sigma$) ion with the He($^1$S) atom, we calculate the relevant state-changing rotationally inelastic collision cross sections from a quantum treatment of the multichannel scattering problem. We focus on the quantum dynamics at the translationally low energies for the present partners discussed in the earlier, cold ion trap experiments (see below) which we wish to model in detail. The corresponding state-changing rates computed between the lower rotational states of the molecular ion are employed to describe the time-evolution kinetics followed by recent experiments on Coulomb-crystalized MgH$^+$ ($^1\Sigma$), where the ions are rotationally cooled by micromotion tuning after the uploading into the trap of He as a buffer gas. The present computational modeling of the final ions' rotational temperatures in the experiments turns out to agree very well with their observations and points at a fast equilibration between rotational and thermal temperatures of the ions.

We present a theoretical study of rotationally elastic and inelastic collisions between molecular nitrogen ions and Rb atoms in the sub-Kelvin temperature regime prevalent in ion-atom hybrid trapping experiments. The cross sections for rotational excitation and de-excitation collisions were calculated using quantum-scattering methods on ab-initio potential energy surfaces for the energetically lowest singlet electronic channel of the system. We find that the rotationally inelastic collision rates are at least an order of magnitude smaller than the charge-exchange rates found in this system, rendering inelastic processes a minor channel under the conditions of typical hybrid trapping experiments., Comment: 6 pages, 5 figures, Computational study of rotational state changing collisions

We employ an accurate, ab initio potential energy surface (PES) which describes the electronic interaction energy between the molecular anion OH$^-$ ($^1\Sigma^+$) and the neutral rubidium atom Rb ($^2S$), to evaluate the elastic and inelastic cross sections over a range of energies representative of the conditions of low-T experiments in MOT traps, when combined with laser-cooled rubidium gas. The system is considered to be in its vibrational ground state, while the first four rotational levels are taken to be involved in the cooling and heating collisional processes that are computed here. The corresponding cooling and heating rates up to about 35 K are obtained from the calculations and compared with the recent results in a similar experiments, where He was the partner atom of the current anion.

This paper deals with the possible formation of two molecular anions often considered likely components in the physical environments of the Interstellar Medium ( ISM) : $\mathrm{C_2H^-}$ and $\mathrm{NCO^-}$. They are both discussed here by computationally following the radiative association (RA) mechanism starting from $\mathrm{C_2^-}$, $\mathrm{H}$, $\mathrm{NC^-}$ and $\mathrm{O}$ as partners. The corresponding RA total cross sections produced by the calculations are in turn employed to generate the overall association rates over the relevant range of temperatures. The latter are found to be in line with other molecular ions formed by RA but not large enough to uniquivocally suggest this path as the main route to the anions formation in the ISM. Other possible paths of formation are also analysed and discussed. The presence of resonant structures during the association dynamics for both systems is found by the calculations and their consequences are discussed in some detail in the present study.

We investigate the relative efficiencies of low-temperature chemical reactions in the Interstellar medium (ISM) with H$^-$ anion reacting in the gas phase with cyanopolyyne neutral molecules, leading to the formation of anionic C$_x$N$^-$ linear chains of different length and of H$_2$. All the reactions turn out to be without barriers, highly exothermic reactions which provide a chemical route to the formation of anionic chains of the same length . Some of the anions have been observed in the dark molecular clouds and in the diffuse interstellar envelopes.Quantum calculations are carried for the corresponding reactive potential energy surfaces (RPESs) for all the odd-numbered members of the series (x=1, 3, 5, 7). We employ the Minimum Energy paths (MEPs) to obtain the relevant Transition State (TS) configurations and use the latter within the Variational Transition State ( VTS) model to obtain the chemical rates. The present results indicate that, at typical temperatures around 100 K, a set of significantly larger rate values exists for x=3 and x=5, while are smaller for CN$^-$ and C$_7$N$^-$. At those temperatures, however, all the rates turn out to be larger than the estimates in the current literature for the Radiative Electron Attachment (REA) rates, thus indicating the greater importance of the present chemical path with respect to REA processes at those temperatures. The physical reasons for our findings are discussed in detail and linked with the existing observational findings.

In the present paper we analyze in detail several properties of the MgH$^-$ anion and the MgH neutral molecule using accurate ab initio quantum computational methods in order to establish with higher reliability specific molecular features like the gas-phase electron affinity (EA) , the Frank-Condon (FC) factors for excitation of the neutral and of its anion to their lower electronic states, and the general feasibility of employing the anion in photodetachment experiments after its confinement in cold ion traps. The calculations suggest that the EA value is in agreement with an existing early experiment and further places on it a smaller error bar than that given before. Accurate zero-point-energy (ZPE) corrections are also included in our calculations and their effects discussed.

We construct the velocity map images of the proton transfer reaction between helium and molecular hydrogen ions H2+. We perform simulations of imaging experiments at one representative total collision energy taking into account the inherent aberrations of the velocity mapping in order to explore the feasibility of direct comparisons between theory and future experiments planned in our laboratory. The asymptotic angular distributions of the fragments in a 3D velocity space is determined from the quantum state-to-state differential reactive cross sections and reaction probabilities which are computed by using the time-independent coupled channel hyperspherical coordinate method. The calculations employ an earlier ab initio potential energy surface computed at the FCI/cc-pVQZ level of theory. The present simulations indicate that the planned experiments would be selective enough to differentiate between product distributions resulting from different initial internal states of the reactants.

(abridged) The fairly recent detection of a variety of anions in the Interstellar Molecular Clouds have underlined the importance of realistically modeling the processes governing their abundance. To this aim, our earlier calculations for the radiative electron attachment (REA) rates for C4H-, C6H-, and C8H- are employed to generate the corresponding column density ratios of anion/neutral (A/N) relative abundances. The latter are then compared with those obtained from observational measurements. The calculations involved the time-dependent solutions of a large network of chemical processes over an extended time interval and included a series of runs in which the values of REA rates were repeatedly scaled. Macroscopic parameters for the clouds' modeling were also varied to cover a broad range of physical environments. It was found that, within the range and quality of the processes included in the present network,and selected from state-of-the-art astrophysical databases, the REA values required to match the observed A/N ratios needed to be reduced by orders of magnitude for C4H- case, while the same rates for C6H- and C8H- only needed to be scaled by much smaller factors. The results suggest that the generally proposed formation of interstellar anions by REA mechanism is overestimated by current models for the C4H- case, for which is likely to be an inefficient path to formation. This path is thus providing a rather marginal contribution to the observed abundances of C4H-, the latter being more likely to originate from other chemical processes in the network, as we discuss in some detail in the present work.Possible physical reasons for the much smaller differences against observations found instead for the values of the (A/N) ratios in two other, longer members of the series are put forward and analyzed within the evolutionary modeling discussed in the present work., Comment: Journal of Physics B, accepted

Chemistry plays a key role in many astrophysical situations regulating the cooling and the thermal properties of the gas, which are relevant during gravitational collapse, the evolution of disks and the fragmentation process. In order to simplify the usage of chemical networks in large numerical simulations, we present the chemistry package KROME, consisting of a Python pre-processor which generates a subroutine for the solution of chemical networks which can be embedded in any numerical code. For the solution of the rate equations, we make use of the high-order solver DLSODES, which was shown to be both accurate and efficient for sparse networks, which are typical in astrophysical applications. KROME also provides a large set of physical processes connected to chemistry, including photochemistry, cooling, heating, dust treatment, and reverse kinetics. The package presented here already contains a network for primordial chemistry, a small metal network appropriate for the modelling of low metallicities environments, a detailed network for the modelling of molecular clouds, a network for planetary atmospheres, as well as a framework for the modelling of the dust grain population. In this paper, we present an extended test suite ranging from one-zone and 1D-models to first applications including cosmological simulations with ENZO and RAMSES and 3D collapse simulations with the FLASH code. The package presented here is publicly available at http://kromepackage.org/ and https://bitbucket.org/krome/krome_stable, Comment: accepted for publication in MNRAS

A detailed quantum analysis of a ionic reaction with a crucial role in the ISM is carried out to generate ab initio reactive cross sections with a quantum method. From them we obtain the corresponding CH+ depletion rates over a broad range of temperatures. The new rates are further linked to a complex chemical network that shows the evolution in time of the CH+ abundance in photodissociation region (PDR) and molecular cloud (MC) environments. The evolutionary abundances of CH+ are given by numerical solutions of a large set of coupled, first-order kinetics equations by employing the new chemical package KROME. The differences found between all existing calculations from low-T experiments are explained via a simple numerical model that links the low-T cross section reductions to collinear approaches where nonadiabatic crossings dominate. The analysis of evolutionary abundance of CH+ reveals that the important region for the depletion reaction of this study is that above 100 K, hence showing that, at least for this reaction, the differences with the existing low-temperature experiments are of essentially no importance within the astrochemical environments. A detailed analysis of the chemical network involving CH+ also shows that a slight decrease in the initial oxygen abundance might lead to higher CH+ abundance since the main chemical carbon ion depletion channel is reduced in efficiency. This simplified observation might provide an alternative starting point to understand the problem of astrochemical models in matching the observed CH+ abundances., Comment: 9 pags, 6 figs, submitted to MNRAS

Chemistry has a key role in the evolution of the interstellar medium (ISM), so it is highly desirable to follow its evolution in numerical simulations. However, it may easily dominate the computational cost when applied to large systems. In this paper we discuss two approaches to reduce these costs: (i) based on computational strategies, and (ii) based on the properties and on the topology of the chemical network. The first methods are more robust, while the second are meant to be giving important information on the structure of large, complex networks. To this aim we first discuss the numerical solvers for integrating the system of ordinary differential equations (ODE) associated with the chemical network. We then propose a buffer method that decreases the computational time spent in solving the ODE system. We further discuss a flux-based method that allows one to determine and then cut on the fly the less active reactions. In addition we also present a topological approach for selecting the most probable species that will be active during the chemical evolution, thus gaining information on the chemical network that otherwise would be difficult to retrieve. This topological technique can also be used as an a priori reduction method for any size network. We implemented these methods into a 1D Lagrangian hydrodynamical code to test their effects: both classes lead to large computational speed-ups, ranging from x2 to x5. We have also tested some hybrid approaches finding that coupling the flux method with a buffer strategy gives the best trade-off between robustness and speed-up of calculations., Comment: accepted for publication in MNRAS

The time-dependent rate coefficients for the charge exchange reaction C+ + Si -> C + Si+ for doublet and quartet states have been determined with ab initio quantum calculations coupled with a non-adiabatic transition model based on a simple Landau-Zener picture. This reaction plays a key role in determining the abundances of C, Si, and their ions, in the ISM since these abundances affect the fine structure cooling and hence the star formation rates. We also provide additional calculations to evaluate the differences between the gas evolution as obtained by using the empirical rate estimates found in the current literature and the calculations presented in this work which are based on our more realistic evaluation of such rates from ab initio transition probabilities . We shall thus show here that the new rates yield important differences for metal-rich environments where $T<10^4$ K and the UV flux is almost negligible, while becoming less important at higher T values and higher photon fluxes., Comment: Accepted to be published on MNRAS

The formation of anionic species in the interstellar medium from interaction of linear molecules containing carbon, nitrogen and hydrogen as atomic components (polyynes) with free electrons in the environment is modelled via a quantum treatment of the collision dynamics. The ensuing integral cross sections are employed to obtain the corresponding attachment rates over a broad range of temperatures for the electrons. The calculations unequivocally show that a parametrization form often employed for such rates yields a broad range of values that turn out to be specific for each molecular species considered, thus excluding using a unique set for the whole class of polyynes., Comment: accepted to be published on MNRAS

CONTEXT: The attachment of free electrons to polycondensed aromatic ring molecules (PAHs) is studied for the variety of these molecules with different numbers of condensed rings and over a broad range of electron temperatures, using a multichannel quantum scattering approach. The calculations of the relevant cross sections are used in turn to model the corresponding attachment rates for each of the systems under study, and these rates are parametrized as a function of temperature using a commonly employed expression for two-body processes in the interstellar medium (ISM). AIM: The scope of this work is to use first principles to establish the influence of chemical properties on the efficiency of the electron-attachment process for PAHs. METHODS: Quantum multichannel scattering methods are employed to generate the relevant cross sections, hence the attachment rates, using integral elastic cross sections computed over a broad range of relevant energies, from threshold up to 1000 K and linking the attachment to low-energy resonant collisions. RESULTS: The rates obtained for the present molecules are found to markedly vary within the test ensemble of the present work and to be lower than the earlier values used for the entire class of PAHs anions, when modelling their evolutions in ISM environments. The effects of such differences on the evolutions of chemical networks that include both PAH and PAH- species are analysed in some detail and related to previous calculations., Comment: accepted to be published on A&A

We present a new computational scheme aimed at reducing the complexity of the chemical networks in astrophysical models, one which is shown to markedly improve their computational efficiency. It contains a flux-reduction scheme that permits to deal with both large and small systems. This procedure is shown to yield a large speed-up of the corresponding numerical codes and provides good accord with the full network results. We analyse and discuss two examples involving chemistry networks of the interstellar medium and show that the results from the present reduction technique reproduce very well the results from fuller calculations., Comment: 9 pages, 7 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

We present a quantum-mechanical study of the exothermic 7LiH reaction with H. Accurate reactive probabilities and rate coefficients are obtained by solving the Schrodinger equation for the motion of the three nuclei on a single Born-Oppenheimer potential energy surface (PES) and using a coupled-channel hyperspherical coordinate method. Our new rates indeed confirm earlier, qualitative predictions and some previous theoretical calculations, as discussed in the main text. In the astrophysical domain we find that the depletion process largely dominates for redshift (z) between 400 and 100, a range significant for early Universe models. This new result from first-principle calculations leads us to definitively surmise that LiH should be already destroyed when the survival processes become important. Because of this very rapid depletion reaction, the fractional abundance of LiH is found to be drastically reduced, so that it should be very difficult to manage to observe it as an imprinted species in the cosmic background radiation (CBR). The present findings appear to settle the question of LiH observability in the early Universe. We further report several state-to-state computed reaction rates in the same range of temperatures of interest for the present problem.

Different computational methods are employed to evaluate elastic (rotationally summed) integral and differential cross sections for low energy (below about 10 eV) positron scattering off gas-phase C$_2$H$_2$ molecules. The computations are carried out at the static and static-plus-polarization levels for describing the interaction forces and the correlation-polarization contributions are found to be an essential component for the correct description of low-energy cross section behavior. The local model potentials derived from density functional theory (DFT) and from the distributed positron model (DPM) are found to produce very high-quality agreement with existing measurements. On the other hand, the less satisfactory agreement between the R-matrix (RM) results and measured data shows the effects of the slow convergence rate of configuration-interaction (CI) expansion methods with respect to the size of the CI-expansion. To contrast the positron scattering findings, results for electron-C$_2$H$_2$ integral and differential cross sections, calculated with both a DFT model potential and the R-matrix method, are compared and analysed around the shape resonance energy region and found to produce better internal agreement.

Calculations have been carried out for the vibrational quenching of excited H$_2$ molecules which collide with Li$^+$ ions at ultralow energies. The dynamics has been treated exactly using the well known quantum coupled-channel expansions over different initial vibrational levels. The overall interaction potential has been obtained from the calculations carried out earlier in our group using highly correlated ab initio methods. The results indicate that specific features of the scattering observables, e.g. the appearance of Ramsauer-Townsend minima in elastic channel cross sections and the marked increase of the cooling rates from specific initial states, can be linked to potential properties at vanishing energies (sign and size of scattering lengths) and to the presence of either virtual states or bound states. The suggestion is made that by selecting the initial state preparation of the molecular partners, the ionic interactions would be amenable to controlling quenching efficiency at ultralow energies.

Photoionization dynamics of the RNA base Uracil is studied in the framework of Density Functional Theory (DFT). The photoionization calculations take advantage of a newly developed parallel version of a multicentric approach to the calculation of the electronic continuum spectrum which uses a set of B-spline radial basis functions and a Kohn-Sham density functional hamiltonian. Both valence and core ionizations are considered. Scattering resonances in selected single-particle ionization channels are classified by the symmetry of the resonant state and the peak energy position in the photoelectron kinetic energy scale; the present results highlight once more the site specificity of core ionization processes. We further suggest that the resonant structures previously characterized in low-energy electron collision experiments are partly shifted below threshold by the photoionization processes. A critical evaluation of the theoretical results providing a guide for future experimental work on similar biosystems.

The low-temperature (up to about 100K) collisional (de)excitation cross sections are computed using the full coupled-channel (CC) quantum dynamics for both Li$_2$ and Li$_2^+$ molecular targets in collision with $^4$He. The interaction forces are obtained from fairly accurate {\it ab initio} calculations and the corresponding pseudo-rates are also computed. The results show surprising similarities between sizes of inelastic flux distributions within final states in both systems and the findings are connected with the structural change in the molecular rotor features when the neutral species is replaced by its ionic counterpart.

Ab initio computed interaction forces are employed in order to describe the microsolvation of the A$_2^+(^2\Sigma)$ (A=Li,Na,K) molecular ion in $^4$He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.

Variational MonteCarlo and Diffusion MonteCarlo calculations have been carried out for cations like Li$^+$, Na$^+$ and K$^+$ as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modelled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab-initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are here discussed in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the clusters features.

Differential cross sections are computed for the title polar molecule using static interaction, exchange forces and correlation-polarisation effects as described in detail in the main text. The dipole effect is also reported via the dipole Born correction procedure and the final angular distributions are compared with existing experimental data. The shape and location of the prominent low-energy resonance are computed and compared with experiments. The comparison shows that the present treatment of the interaction forces and of the quantum dynamics can indeed afford good agreement between measured and computed quantities for a multielectron target as OCS.

A structural study of the smaller Li$^+$He$_n$ clusters with $n\le30$ has been carried out using different theoretical methods. The structures and the energetics of the clusters have been obtained using both classical energy minimization methods and quantum Diffusion Monte Carlo. The total interaction acting within the clusters has been obtained as a sum of pairwise potentials: Li$^+$-He and He-He. This approximation had been shown in our earlier study \cite{8} to give substantially correct results for energies and geometries once compared to full ab-initio calculations. The general features of the spatial structures, and their energetics, are discussed in details for the clusters up to $n=30$ and the first solvation shell is shown to be essentially completed by the first ten helium atoms.

The potential energy surface (PES) describing the interactions between $\mathrm{Li_{2}(^{1}\Sigma_{u}^{+})}$ and $\mathrm{^{4}He}$ and an extensive study of the energies and structures of a set of small clusters, $\mathrm{Li_{2}(He)_{n}}$, have been presented by us in a previous series of publications [1-3]. In the present work we want to extend the same analysis to the case of the excited $\mathrm{Li_{2}}(a^{3}\Sigma_{u}^{+})$ and of the ionized Li$_{2}^{+}(X^{2}\Sigma_{g}^{+})$ moiety. We thus show here calculated interaction potentials for the two title systems and the corresponding fitting of the computed points. For both surfaces the MP4 method with cc-pV5Z basis sets has been used to generate an extensive range of radial/angular coordinates of the two dimensional PES's which describe rigid rotor molecular dopants interacting with one He partner.

We present quantum calculations of the rotational energy levels and spectroscopic rotational constants of the linear OCS molecule in variable size clusters of 4He, using spectral evolution quantum Monte Carlo methods that allow excited states to be accessed without nodal constraints. The rotational constants of OCS are found to decrease monotonically from the gas phase value as the number of helium atoms increases to N=6, after which the average constant increases to saturation at a value in excellent agreement with experimental measurements made on significantly larger clusters (N>1000). The minimum is shown to indicate a transition from a molecular complex to a quantum solvated molecule, with the former characterized by floppy but near rigid behavior, while the latter is characterized by non-zero permutation exchanges and a smaller extent of rigid coupling., Comment: 4 pages, 1 figure, submitted to Phys. Rev. Lett. on July 18,2002