M. F. Falcetta

Application of the stabilization method to the N−2(1 2Πg) and Mg−(1 2P) temporary anion states

A new variant of the stabilization method is described and used in conjunction with an analytic continuation procedure to calculate the energies and lifetimes of the lowest energy temporary anion states of N2 and Mg. With this approach excellent results are obtained for the resonance parameters even when moderate size basis sets are employed.

Assessment of various electronic structure methods for characterizing temporary anion states: application to the ground state anions of N2, C2H2, C2H4, and C6H6.

The theoretical characterization of temporary anions is an especially challenging problem. In the present study we assess the performance of several electronic structure methods when used in conjunction with the stabilization method to characterize temporary anion states. The ground state anions of N2, C2H2, C2H4, and C6H6 are used as the test systems, with the most extensive testing being done for N2. For the (2)Πg anion state of N2(-) the ADC(2), EOM-MP2, and EOM-CCSD methods give values of the resonance parameters in excellent agreement with the results of prior high-level calculations. For the hydrocarbon systems, the EOM-MP2 method consistently provides excellent agreement with the EOM-CCSD results for the test systems, whereas the ADC(2) considerably underestimates the widths for ethylene and benzene. Several density functional theory (DFT) approaches are tested and, although none performs as well as the EOM-MP2 method, it is found that inclusion of Hartree-Fock exchange greatly improves the results. Of the DFT-based methods, time-dependent DFT with standard correlation functionals and use of Hartree-Fock exchange provides the best performance for N2(-) over the range of bond lengths considered and is also found to give reasonable values of the resonance parameters of the three hydrocarbon molecules.

Ab initio/spectroscopic interaction potential for He+Ne+

High-level ab initio calculations have been carried out on the lowest Σ and Π states of HeNe+. These have been used to construct a new interaction potential in a Hund’s case (e) representation, by fitting spectroscopic vibrational spacings ΔGv+1/2 and rotational constants Bv using a close-coupling method and a potential function whose form is established by the ab initio data and a long-range analysis. The characteristics of the resulting Born–Oppenheimer potential curves, particularly for the X state, where only the higher vibrational levels were observed, differ considerably from those derived by extrapolation of the experimental spectroscopic constants. A new set of constants is proposed, and functions for the X-state G(v) and Bv are given that are well-behaved from the bottom of the well to the dissociation limit. The asymptotic formula for Bv of Le Roy is extended to improve its accuracy. The X state of 4HeNe+ is predicted to support 15 bound vibrational states, the A2 state 7. Good agreement with ex...

THEORETICAL STUDY OF ION-MOLECULE POTENTIALS FOR HE+ AND LI+ WITH N2

High-level ab initio calculations have been carried out on the lowest charge-transfer excited state of [HeN2]+ and the [LiN2]+ ground state, over a range of intermolecular distances R of 3–20a0 at a fixed N2 bond length re=2.074 30a0 for three orientation angles γ=0°, 45°, and 90°. The calculations employed extended atomic basis sets, chosen to represent accurately the electrical properties of the interacting partners; for N2 the key properties α∥, α⊥, and Θ are within 1.7%, 2.7%, and 2.1% of the best experimental values. All interaction energies were corrected for basis-set superposition error by the counterpoise method, and fitted by analytic forms incorporating the proper long-range expansion through R−7. Our value for the tetrahexacontapole (26-pole) moment of N2 is −15.95 a.u. The most stable geometries for both systems occur for linear (γ=0°) complexes, with minima −De of −7.00 (−12.65) kcal/mol located at Re=3.048 (2.610) A for He+(Li+)+N2; the Li+ values are in good agreement with previous theoret...

Ab initio study of the long-range interaction between He+ and H2

Abstract A new estimate of the anisotropic long-range potential energy surface for the interaction of He + (1s 2 S) and H 2 (X 1 Σ + g ) has been computed using extended atomic basis sets to construct three-configuration self-consistent-field (MCSCF) molecular orbitals for use in a multireference configuration interaction expansion including all single and double excitations (MRCISD). The most stable geometry is C 2v , in agreement with earlier studies, but the well depth is found to be 3.15 kcal/mol at an intermolecular distance of 2.42 A, 57% deeper and 0.2 A smaller, respectively, than the best previous estimate. The ab initio points are fitted to a potential function with correct long-range behavior, suitable for scattering calculations.

Ab initio calculation of the cross sections for electron impact vibrational excitation of CO via the 2Π shape resonance

The stabilization method is used to calculate the complex potential energy curve of the (2)Π state of CO(-) as a function of bond length, with the refinement that separate potentials are determined for p-wave and d-wave attachment and detachment of the excess electron. Using the resulting complex potentials, absolute vibrational excitation cross sections are calculated as a function of electron energy and scattering angle. The calculated cross sections agree well with experiment.