The valence shell binding energy spectra and orbital electron momentum profiles of O-2 have been measured by energy dispersive multichannel electron momentum spectroscopy at an impact energy of 1200 eV + binding energy. The effects of electron correlation on the valence binding energy spectrum are investigated using multi-reference singles and doubles configuration interaction calculations. The presently reported experimental momentum profiles of O-2 display considerably improved statistics compared with previously published EMS results. The measured momentum profiles are compared with cross sections calculated using both unrestricted and restricted open shell Hartree-Fock methods with basis sets ranging from minimal to near Hartree-Fock limit in quality. In addition, the effects of correlation and relaxation on the calculated momentum profiles are investigated using multi-reference singles and doubles configuration interaction calculations of the full ion-neutral overlap distributions. Electron correlation effects in the ground state are further examined using several density functional approaches for the momentum profiles. The present EMS measurements and MRSD-CI calculations clearly show that the binding energy peak at similar to 27.3 eV has significant contributions from both (4) Sigma(u)(-) and (2) Sigma(u)(-) processes in contrast to earlier assignments which have attributed this peak to the C-2 Sigma(u)(-) State alone. Similarly, the binding energy peak at 33 eV is shown to be due to (2) Sigma(u)(-) rather than earlier assignments of (2) Pi(u) character. (C) 1998 Elsevier Science B.V. All rights reserved.

}, keywords = {CALCULATIONS, CONFIGURATION-INTERACTION CALCULATIONS, CORRELATED MOLECULAR, CORRELATION ENERGIES, DENSITY-FUNCTIONAL THEORY, DFT calculations, GAUSSIAN-BASIS SETS, HARTREE-FOCK LIMIT, OPEN-SHELL MOLECULES, PHOTOELECTRON-SPECTROSCOPY, PHOTOIONIZATION CROSS-SECTIONS}, isbn = {0301-0104}, url = {Measurements of the outermost valence electron orbital momentum profiles of the open shell molecules NO, O-2 and NO2 have been obtained using electron momentum spectroscopy (EMS). The presently reported experimental momentum profiles of NO and O-2 display much improved statistics compared with previously published EMS results while the data for the HOMO of NO, is the first reported. In the case of NO, the present measurements are considerably different from previous results and these differences appear to be due to the presence of NO2 impurities in the previous work. The EMS measurements provide a stringent test of basis set effects and the quality of ab initio methods in the description of these open shell systems. The experimental momentum profiles have been compared with theoretical spherically averaged momentum profiles from several basis sets calculated at the level of the target Hartree-Fock approximation (THFA) with a range of basis sets using both unrestricted Hartree-Fock (UHF) and restricted open shell Hartree-Fock (ROHF) methods. Various configuration interaction calculations such as multi-reference singles and doubles configuration interaction (MRSD-CI), averaged coupled pair functional (ACPF) and quasi-degenerate variational perturbation theory (QDVPT) calculations of the full ion-neutral overlap amplitude have also been compared to experiment to investigate the effects of electron correlation and relaxation. The experimental momentum profiles have further been compared to calculations at the level of the target Kohn-Sham approximation (TKSA) using density functional theory (DFT) with the local density approximation and also with gradient corrected exchange-correlation potentials. In addition to momentum profiles, other electronic properties such as total energies, dipole moments, quadrupole moments and values of the electronic spatial extent have been calculated by the various theoretical methods and compared to experimental values.

}, keywords = {CORRELATED WAVEFUNCTIONS, CORRELATION ENERGIES, density, DIPOLE-MOMENT, EXCHANGE-ENERGY, GAUSSIAN-BASIS SETS, NITRIC-OXIDE, VALENCE ORBITALS, VARIATIONAL PERTURBATION-THEORY, WAVE-FUNCTIONS}, isbn = {0301-0104}, url = {