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The spectroscopic parameters and separations between the three low-lying X 3Πu, A 3Σ-g, and a 1Σ+g states of Al2 are studied as a function of both the one-particle and n-particle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3Πu–3Σ-g separation is in excellent agreement with the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/[6s5p3d2f] Gaussian basis. Including a small correction for relativistic effects, our best estimate is that the 3Σ-g state lies 174 cm-1 above the 3Πu ground state. The 1Σ+g state lies at least 2000 cm-1 higher in energy. At the CPF level, inclusion of 2s and 2p correlation has little effect on De, reduces Te by only 26 cm-1, and shortens the bond lengths by about 0.02 a0. Further strong support for a 3Πu ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3Πu→3Πg. The (2)3Πg state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter r values, such that the strongest experimental absorption peak probably corresponds to the 0→2 transition. [ABSTRACT FROM AUTHOR]

Theoretical potentials for the 7Σ+u state are reported using both extended Slater and Gaussian basis sets. Electron correlation is included using the interacting correlated fragments (ICF), the singles plus doubles configuration-interaction (SDCI), and the coupled-pair functional (CPF) approaches. Our best potential, corrected for basis set superposition errors, has a well depth of about 21 cm-1 and an re of 7.52 bohr. The inclusion of the nitrogen 2s correlation significantly increases the well depth. These results are in reasonably good agreement with the empirical potential of Ferrante and Stwalley, and support the contention that spin-polarized atomic nitrogen should behave like a classical solid such as Ne. It is shown that the modified Buckingham potential used by Ferrante and Stwalley is consistent with our ab initio potentials if smaller values are used for the dispersion coefficients of nitrogen atom. [ABSTRACT FROM AUTHOR]

Theoretical spectroscopic parameters are presented at the self-consistent-field and singles-plus-doubles configuration-interaction (SDCI) levels for the ground states of the positive ions of the alkaline-earth fluorides, oxides, and hydroxides. The alkaline-earth fluoride and hydroxide ions are found to have X 1Σ+ ground states corresponding to a M+2X-1 structure. The alkaline-earth oxide positive ions, on the other hand, undergo a change in ground state symmetry between CaO+(X 2Π) and SrO+(X 2Σ+), whereas the analogous alkali oxides undergo the change one row higher between NaO and KO. The greater stability of the 2Π states for the alkaline-earth oxide positive ions is due to the increased electrostaticfield created by the larger metal ion charge and reduced bond lengths. Calculations on the 3Π states of BeF+, MgF+, BeOH+, and MgOH+ indicate that these states, corresponding to the M+1X structure, lie much higher in energy. The bond distances of the positive ions are uniformly 0.07±0.02 Å shorter than the analogous electronic states of the neutral. By correcting for the relatively small errors in the theoretical bond distances for the neutrals, bond distances accurate to about 0.005 Å are obtained for the ions where experimental values are available for the neutral species. Since extensive one-particle basis sets are employed and differential correlation effects are minimized by dissociating directly to ions, theoretical dissociation energies and ionization potentials (I.P.) accurate to about 0.15 eV can be obtained. The dissociation energies (D0) in eV for the ground states at the SDCI level are found to be: BeO+(4.04), MgO+(2.31), CaO+(3.29), SrO+(3.28), BeF+(6.25), MgF+(4.46), CaF+(5.67), SrF+(5.71), BeOH+(6.00), MgOH+(3.62), CaOH+(4.73), and SrOH+(4.67). For all of the systems studied, the bonding is dominated by electrostatic interactions. However, the molecules containing Be show some covalent character owing to... [ABSTRACT FROM AUTHOR]

The lowest 5Π and 5Σ+ states of CO are characterized using very large one-particle basis sets and the internally contracted multireference configuration–interaction treatment of electron correlation. It is found that the (1)5Σ+ state is weakly bound (De=572 cm-1) with an re of 4.76 a0, in good agreement with earlier theoretical calculations by Rosenkrantz et al. [Theor. Chim. Acta 82, 153 (1992)]. The (1)5Π state (hereafter designated a‘ 5Π) has a weakly bound outer well (De=18 cm-1, re=5.80 a0) and a significantly bound inner well (De=6043 cm-1, re=2.76 a0). It is shown that the emission spectrum between about 360 and 520 nm observed by Bahrdt et al. [Chem. Phys. 144, 273 (1990)] can be assigned to the a‘ 5Π-a’ 3Σ+ and a‘ 5Π-d 3Δ transitions. Franck–Condon factors are presented to aid in the experimental identification of the a‘ 5Π state by absorption from the low-lying triplet states of CO. [ABSTRACT FROM AUTHOR]

Theoretical spectroscopic constants are reported for the X 1Σ+ and a 3Δ states of the fluorides, chlorides, and bromides of scandium and yttrium. The calculated D0 values (eV) are ScF(6.00), ScC1(4.55), ScBr(3.90), YF(6.72), YC1(5.36), and YBr(4.74). All systems are predicted to have 1Σ+ ground states once the effect of unlinked higher excitations are incorporated using the coupled-pair functional approach and relativistic effects are included using either perturbation theory or relativistic effective core potentials. The singlet–triplet splitting is larger for the yttrium halides and decreases for both metals from F to Br. A complete-active space self-consistent-field plus multireference configuration-interaction study of the spectroscopic constants and radiative lifetimes is presented for most of the singlet and triplet states of ScF and YCl below 28 000 cm-1. The theoretical Te value computed for the B 1Π state of ScF indicates that the longest wavelength peak (9405 Å) for the B 1Π←X 1Σ+ system observed in absorption in a neon matrix probably corresponds to the 2–0 band. The calculations provide an unambiguous assignment of two previously unassigned band systems in the laser excitation spectrum of the YCl molecule. [ABSTRACT FROM AUTHOR]

Spectroscopic parameters (De,re,μe) are determined for the second-row transition metal hydrides using large valence basis sets in conjunction with relativistic effective core potentials (RECPs). All-electron calculations are also performed for YH and AgH to calibrate the RECP results. Electron correlation is incorporated using singles-plus-doubles configuration interaction (SDCI), the coupled pair functional (CPF) method, and a modified version (MCPF) of CPF. Although similarities exist between the bonding in the first- and second-row transition metal hydrides, the greater overlap of the d orbitals in the second row with the hydrogen 1s orbital, tends to lead to larger dissociation energies and some changes in the relative ordering of the states. For example, the ground state of ZrH is predicted to be a 2Δ state whose bonding involves 4d–5s hybrid orbitals, whereas in TiH the ground state is a 4[uppercase_phi_synonym] state with primarily 4s–1s bonding. The bonding in the second-row transition metal hydrides involves a mixture of all three atomic asymptotes, 4dn5s2, 4dn+15s1, and 4dn+2, whereas contribution from the 3dn+2 asymptote is unimportant in the first-row TM hydrides. However, the bonding is generally much simpler to describe in the second-row as compared with the first-row TM hydrides, and the spectroscopic parameters are much less sensitive to the level of correlation treatment. [ABSTRACT FROM AUTHOR]

Ab initio calculations at the self-consistent field and singles plus doubles configuration-interaction level are used to determine accurate spectroscopic parameters (De,re,ωe) for the 2Π and 2∑+ states of the alkali oxides and the a 3Π (or A 1Π) states of the alkaline-earth oxides. Numerical Hartree–Fock (NHF) calculations performed on KO demonstrate that the extended Slater basis sets employed are near the Hartree–Fock limit. When the dissociation is referenced to the ionic limits, the differential correlation contribution is found to increase the D0 by a relatively small amount (0.0–0.2 eV). An accurate description of the alkali and alkaline-earth oxides requires correlating both the oxygen and metal electrons. The theoretical dissociation energies (D0) permit a critical assessment of the experimental literature and allow us to recommend D0 values that are accurate to 0.1 eV for all systems considered. There is a strong correlation between the dissociation energy (to ions) and re, because the bonding is predominantly electrostatic in origin. Theoretical 2Π–2∑+ energy separations are presented for the alkali oxides. An extensive study of the 2Π–2∑+ energy separation in KO reveals a 2∑+ ground state at all levels of theory. Basis set studies in combination with NHF calculations indicate different basis set requirements for the 2Π and 2∑+ states. In the NHF limit the 2∑+ state of KO is lower by about 250 cm-1. The separation is almost unaffected when the 15 valence electrons are correlated at the singles plus doubles level using an extended Slater basis. [ABSTRACT FROM AUTHOR]

Ab initio calculations using self-consistent-field and correlated wave functions are used to determine accurate spectroscopic parameters (re, ωe, D0) for the alkali and alkaline-earth monofluorides and monochlorides. Numerical Hartree–Fock (NHF) calculations are performed on selected systems to ensure that the extended Slater basis sets employed are near the Hartree–Fock limit. Because the bonding is predominantly electrostatic in origin, there is a strong correlation between the dissociation energy (to ions) and re. By dissociating to the ionic limits, most of the differential correlation effects can be embedded in the accurate experimental electron affinities and ionization potentials. With this model, correlation effects are relatively small (0.0–0.4 eV), but invariably increase D0. The importance of correlating the electrons on both the anion and the metal is discussed. For the heavier alkali chlorides, RbCl and CsCl, we show that a core-valence treatment, which excludes the double excitations out of the metal (n-1) shell, gives significantly better re and D0 parameters than does the full singles plus doubles configuration-interaction CI(SD) calculation. The theoretical studies combined with the experimental literature allow us to recommend D0 values that are accurate to 0.1 eV for all systems considered. [ABSTRACT FROM AUTHOR]

The singlet and triplet states of AlCu and AlAg below about 32 000 cm-1 are studied using the internally contracted multireference configuration-interaction method. A more elaborate study of the X 1Σ+ ground state of AlCu is undertaken using extended Gaussian basis sets, including the effect of inner-shell correlation and including a perturbational estimate of relativistic effects. Our best estimate of the spectroscopic constants (r0, ΔG1/2, and D0) for the X 1Σ+ state with the experimental values in parentheses are: 4.416(4.420) a0, 295(294) cm-1, and 2.318(2.315) eV. The calculations definitively assign the upper state in the observed transition at 14 892 cm-1 to the lowest 1Π state. The calculated spectroscopic constants and radiative lifetime for the 1Π state are in good agreement with experiment. The calculations support the tentative assignments of Behm et al. for three band systems observed in the visible region between 25 000 and 28 000 cm-1. However, the computed spectroscopic constants are in very poor agreement with those deduced from an analysis of the spectra. Analogous theoretical results for AlAg suggest that the (2)3Π, (3)3Π, and (3)1Σ+ states account for the bands observed, but not assigned, by Duncan and co-workers. [ABSTRACT FROM AUTHOR]