Search

Continuum photoabsorption of lithium chloride (LiCl) was investigated using a coupled-channel time-dependent wave packet approach. Photodissociation cross sections for the production of ground-state Li and Cl atoms were computed up to temperatures of 1500 K for a thermal distribution of rotational levels. At such temperatures, LiCl is believed to be the primary Li-bearing gas in cool stellar atmospheres. Narrow Rydberg resonances in the total absorption spectrum are found to dominate the thermally averaged cross section due to the large density of Rydberg states in the predissociation gap. Comparison with measured photoabsorption cross sections, where available, is made. [ABSTRACT FROM AUTHOR]

Ab initio configuration interaction calculations have been performed for the X [sup 1]Σ[sup +] and B [sup 1]Σ[sup +] electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R=2.5a[sub 0] and 50a[sub 0]. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R=16.2a[sub 0], in close agreement with previous calculations using a semiempirical covalent–ionic resonance model. X [sup 1]Σ[sup +] state spectroscopic constants are in excellent agreement with experimental values.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Comprehensive absorption line lists for [SUP40]CaH have been calculated for electronic transitions from the X [SUP2]∑[SUP+] ground state to the A [SUP2]Π, B/B' [SUP2]∑[SUP+], and E [SUP2]Π low-lying excited states. The lists include transition energies and oscillator strengths for all possible allowed transitions and were computed using the most recent set of theoretical potential energy curves and dipole transition moment functions, with adjustments to account for experimental dissociation energies and asymptotic limits. Good agreement with previous calculations and available experimental data has been obtained. Oscillator strengths for the transition from the X [SUP2]∑[SUP+] state to the D [SUP2]∑[SUP+] state are also given, but due to the large uncertainty of the available dipole transition moment function the line list is highly uncertain. For the C [SUP2]∑[SUP+]← X [SUP2]∑[SUP+] transition, a Franck--Condon approximation has been used. [ABSTRACT FROM AUTHOR]

Ab initio calculations of electronic wave functions and potential energy curves for each of two low-lying states of 1Σ- and 1Δ symmetry for CO are reported. The second state of each symmetry has Rydberg character. Computed spectroscopic constants for the metastable I 1Σ- and D 1Δ states appear to be in good agreement with experimental data. The radiative lifetimes of vibrational levels within the I 1Σ- and D 1Δ states have been computed considering two types of decay: cascade within each electronic state, and transitions to the nearby A 1Π. It is shown that the v=0 levels of each metastable electronic state have very long lifetimes. The observation of these metastable states in electron impact experiments is discussed in light of our results. [ABSTRACT FROM AUTHOR]

The charge transfer reaction Al3++H→Al2++H+ has been studied within a molecular representation appropriate for slow to intermediate velocity collisions. Ab initio electronic potential energy curves were computed for the lowest four 2Σ+ states of AlH3+, which dissociate to the limits Al3++H and Al2+(2S,2P0,2D)+H+. The configuration interaction wave functions were constructed to represent the four states in an unbiased fashion yielding asymptotic energy splittings within 1% of experimental values. Radial and dipole coupling matrix elements were obtained as a function of internuclear separation. Two formal criteria are proposed to evaluate the accuracy of the radial coupling elements which, in this case, were obtained through the Hellman–Feynman theorem. The nuclear scattering equations were solved in the semiclassical straight line trajectory approximation for the range of barycentric collision energies 0.01–1.0 keV/amu. [ABSTRACT FROM AUTHOR]

The recent boom in computational chemistry has enabled several projects aimed at discovering useful materials or catalysts. We acknowledge and address two recurring issues in the field of computational catalyst discovery. First, calculating macro-scale catalyst properties is not straightforward when using ensembles of atomic-scale calculations [e.g., density functional theory (DFT)]. We attempt to address this issue by creating a multi-scale model that estimates bulk catalyst activity using adsorption energy predictions from both DFT and machine learning models. The second issue is that many catalyst discovery efforts seek to optimize catalyst properties, but optimization is an inherently exploitative objective that is in tension with the explorative nature of early-stage discovery projects. In other words, why invest so much time finding a "best" catalyst when it is likely to fail for some other, unforeseen problem? We address this issue by relaxing the catalyst discovery goal into a classification problem: "What is the set of catalysts that is worth testing experimentally?" Here, we present a catalyst discovery method called myopic multiscale sampling, which combines multiscale modeling with automated selection of DFT calculations. It is an active classification strategy that seeks to classify catalysts as "worth investigating" or "not worth investigating" experimentally. Our results show an ∼7-16 times speedup in catalyst classification relative to random sampling. These results were based on offline simulations of our algorithm on two different datasets: a larger, synthesized dataset and a smaller, real dataset.

The potassium resonance line (4s-->4p) centered around 770 nm is a major contributor to the optical extinction in the atmospheres of certain classes of brown dwarfs and extrasolar giant planets. The resonance line is significantly broadened by collisions with He and H2, and an accurate calculation of the line profile is needed for astrophysical models of these objects. As a first step, we report an accurate ab initio study of the K+He and K+H2 potential-energy curves correlating to the K 4s and 4p atomic energy levels, together with the dipole moments governing the transitions between these potential-energy curves. The molecular calculations have been carried out using a multireference configuration-interaction method, with the molecular orbitals expanded in a large Gaussian basis set. The transition dipole moments show significant variation with the molecular geometry. Calculations for the K+H2 system have been carried out for a range of H2 orientations and internuclear separations, so that the effect of H2 rotation and vibration may be explicitly included in future calculations of the pressure-broadened line profiles.