Franco A. Gianturco

KROME – a package to embed chemistry in astrophysical simulations

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 this http URL and this https URL

Ring-breaking electron attachment to uracil: Following bond dissociations via evolving resonances

Calculations are carried out at various distinct energies to obtain both elastic cross sections and S-matrix resonance indicators (poles) from a quantum treatment of the electron scattering from gas-phase uracil. The low-energy region confirms the presence of pi(*) resonances as revealed by earlier calculations and experiments which are compared with the present findings. They turn out to be little affected by bond deformation, while the transient negative ions (TNIs) associated with sigma(*) resonances in the higher energy region ( approximately 8 eV) indeed show that ring deformations which allow vibrational redistribution of the excess electron energy into the molecular target strongly affect these shape resonances: They therefore evolve along different dissociative pathways and stabilize different fragment anions. The calculations further show that the occurrence of conical intersections between sigma(*) and pi(*)-type potential energy surfaces (real parts) is a very likely mechanism responsible for energy transfers between different TNIs. The excess electron wavefunctions for such scattering states, once mapped over the molecular space, provide nanoscopic reasons for the selective breaking of different bonds in the ring region.

Classical trajectory calculations of transport and relaxation properties for Ar–N2 mixtures

Classical trajectory calculations of transport and relaxation properties have been performed for Ar–N2 mixtures using the potential energy surface (PES) recently determined by Bowers et al. [J. Chem. Phys. 88, 5465 (1988)]. Generalized cross sections have been computed in the temperature range 77.3–1000 K. Extensive comparisons have been carried out with available measurements and with other calculations. The present system exhibits greater efficiency for rotational energy transfer (RET) processes and its interaction shows a deeper potential well than that of previously computed surfaces. A larger number of trajectories (up to 28 500 at the lowest total energy examined) has therefore been required to obtain converged results. The PES employed here shows impressive agreement with the available measurements for a wide variety of properties of the system and appears to be the most reliable currently available for Ar–N2 gaseous mixtures.

Bosonic helium droplets with cationic impurities : Onset of electrostriction and snowball effects from quantum calculations

Variational Monte Carlo and diffusion Monte Carlo calculations have been carried out for cations such as 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 modeled 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 discussed here in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the cluster features.

Computed transport coefficients for van der Waals systems via realistic interactions

Approximate methods are employed to calculate generalized collision integrals for gas phase rotationally inelastic processes in mixtures of N2 with He, Ne and Ar. A detailed comparison is performed between theoretical diffusion and viscosity coefficients and the available measurements at various temperatures. Because of the relatively weak interactions involved, by using several existing potential energy surfaces one can specifically relate the behaviour of the transport coefficients to the ‘shape’ and ‘size’ of the averaged, effective interactions which are probed by these properties. For each of the systems examined the most reliable potential energy surface is therefore selected and recommended for calculations.

ON THE RELATIVE ABUNDANCE OF LiH AND LiH+ MOLECULES IN THE EARLY UNIVERSE: NEW RESULTS FROM QUANTUM REACTIONS

The relative efficiencies of the chemical pathways that can lead to the destruction of LiH and LiH+ molecules, conjectured to be present in the primordial gas and to control molecular cooling processes in the gravitational collapse of the post-recombination era, are revisited by using accurate quantum calculations for the several reactions involved. The new rates are employed to survey the behavior of the relative abundance of these molecules at redshifts of interest for early universe conditions. We find significant differences with respect to previous calculations, the present ones yielding LiH abundances higher than LiH+ at all redshifts.

Collisional quenching of molecular ro-vibrational energy by He buffer loading at ultralow energies

In the present review we summarize in some detail the ample theoretical literature which has discussed over the last few years the computational treatment of collisionally inelastic processes at ultralow temperatures. The analysis is centred on the ab initio quantum treatment of collisional quenching of ro-vibrational states of simple diatomics, neutral and ionic, polar and homonuclear, interacting with a helium buffer gas. Several specific examples are analysed and their features are linked with both the details of their interaction potential energy surfaces and the special behavior of ultralow energy quantum dynamics. †This work is affectionately dedicated to the late Roger Miller, a brilliant scientist and a dear friend whose early departure has left a great void in our midst. Contents page 1. Introduction 314 2. The theoretical machinery 317  2.1. Zeeman depolarization 319  2.2. Ultralow energy collisions 319 3. Rotational quenching 321  3.1. Neutral systems with closed-shell molecules 322  3.2. Neutral systems with open-shell molecules 326  3.3. Ionic systems 329 4. Vibrational quenching 334  4.1. Quenching from low ν states 335  4.2. Quenching from high ν states 340  4.3. Ionic systems: virtual state scattering 343 5. Present conclusions 348 Acknowledgments 348 References 348

Classical and quantum dynamics on the collinear potential energy surface for the reaction of Li with H2

Abstract Extensive Valence Bond (VB) calculations using non-orthogonal, spin-coupled wavefunctions with optimized orbitals have been employed to analyse the reactive region for the title process. Both the exothermic channel and the reverse, endothermic process are of strong astrophysical interest although no previous calculations have been available on both the reactive dynamics and the interaction energy surface. The specific features of the potential are analysed for some indicative configurations and classical trajectory calculations are carried out for the special collinear arrangement. In the latter instance, quantum time-dependent wavepacket calculations have also been performed and the two sets of results are found to be in rather good accord with each other. Some consequences of these exploratory calculations are discussed in detail.

SELECTIVE EFFICIENCY OF VIBRATIONAL EXCITATIONS IN ION-MOLECULE COLLISIONS: A COMPARISON OF BEHAVIOR FOR H+-H2 AND H--H2

The vibrational excitation processes which occur in molecular beam experiments on H2 molecules, and using H+ or H− as projectiles, are discussed from the theoretical viewpoint of the microscopic quantum dynamics and in relation to the various features of the two potential energy surfaces. The present study employs the vibrational close‐coupling–rotational infinite‐order sudden (VCC–RIOS) decoupling scheme and analyzes in detail the differences of behavior of the various inelastic differential cross sections in the small‐angle region. It is clearly found that two separate mechanisms can be invoked in the two systems to explain the differences in efficiency between the two excitation processes. Such mechanisms can be related in turn to specific features of the two potential energy surfaces and to their bearing on the final dynamical observables. Rather good agreement between calculated and observed cross sections is found for both systems.

FAST LiH DESTRUCTION IN REACTION WITH H: QUANTUM CALCULATIONS AND ASTROPHYSICAL CONSEQUENCES

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 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. 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.