Core Electron Correlation Research Articles

FTMW spectroscopy of the NC 2O and NC 3O radicals and ab initio calculations

Rotational transitions of the carbon-chain radicals, NC 2O and NC 3O, have been observed for the first time by Fourie-transform microwave spectroscopy. The radicals were produced in a supersonic free jet by a pulsed discharge of acetylcyanide (CO(CH 3)(CN)) diluted in Ar for NC 2O or a gas mixture of HC 3N and O 2 diluted in Ar for NC 3O. Pure rotational transitions from N=1–0 to N=3–2 for NC 2O and those from N=2–1 to N=6–5 for NC 3O, both with fine and hyperfine structures, were observed. The rotational, spin-rotation, and hyperfine coupling constants for the radicals have been precisely determined. Ab initio calculations at the RCCSD(T) level of theory considering the correlation of core electrons using the cc-pCVTZ basis sets have been performed. The present observations and the ab initio calculations revealed that both the radicals have bent structures in the ground states.

The ab initio potential energy surface and vibrational-rotational energy levels of X 2Σ+ MgOH

The equilibrium structure and potential energy surface of magnesium monohydroxide in its ground doublet state, X 2Σ+ MgOH, have been determined from large-scale ab initio calculations using the spin-restricted coupled-cluster method, RCCSD(T), with basis sets of double-through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The vibrational-rotational energy levels of various MgOH isotopomers were calculated using the variational method. The spectroscopic constants determined are found to be in remarkably good agreement with experimental data.

Spin–spin coupling constants in ethylene: equilibrium values

The nuclear magnetic resonance spin–spin coupling constants for the ethylene have been calculated at the multiconfigurational self-consistent field (MCSCF) method using the restricted (RAS) and complete (CAS) active space approximations. An analysis of the calculated values and of the effects of different factors (active spaces, valence and core–electron correlation) is presented. The effect of increase the number of active orbitals and that of triple and quadruple excitations is important and opposite to the effect of core–electron correlation. The best calculated values for 1J CC (68.83 Hz), 1J CH (151.56), 2J CH (−1.56 Hz), 2J HH (1.07 Hz) 3J HH (cis) (11.47 Hz), and 3J HH (trans) (17.78 Hz) are in good agreement with the best calculated and experimental values.

Theab initiopotential energy surface and vibrational–rotational energy levels of dilithium monoxide, Li2O

The equilibrium structure and potential energy surface of dilithium monoxide, Li2O, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double- through quintuple-zeta quality. The effects of core–electron correlation on the calculated molecular parameters were investigated. The vibrational–rotational energy levels of the LiO77Li and LiO76Li isotopic species were calculated by a variational approach. A comparison with results of recent experimental high-resolution studies is presented.

Fine-structure splittings in2Fstates of rubidium via three-step laser spectroscopy

Three-step laser excitation is employed to measure the ${}^{2}{F}_{5/2}{\ensuremath{-}}^{2}{F}_{7/2}$ fine-structure splittings in seven $n{}^{2}F$ manifolds of Rb for $4<~n<~10.$ The use of cycling transitions in the excitation process intensifies the spectra and simplifies signal detection. This sequence of seven measured splittings should serve as a useful test of fine-structure calculations in heavy alkali-metal atoms where complicated, noncentral field treatments of core polarization, configuration mixing, electron correlation, and/or relativistic effects are generally required.

Raman and infrared spectra, conformational stability, ab initio calculations and assignment of fundamentals for 1-bromo-3-fluoropropane

The infrared and Raman spectrum of 1-bromo-3-fluoropropane is reported in the gas, liquid, amorphous solid and annealed polycrystalline states. Only one of the five possible conformers is stable in the crystal, designated the C conformer. The disordered phases show the presence of several other conformers of higher energy, due entirely to conformers designated B and D. Ab initio calculations were performed as rhf/4-31g*/MIDI-4*, rhf/6-31g* and mp2/6-31g* (both frozen core and full electron correlation) for all five conformers. The scaled harmonic force field obtained using the mp2=full/6-31g* level of the theory is reported for the most stable conformer together with an assignment of fundamentals and potential energy distributions for local symmetry coordinates. Selected computational results are reported for all conformers together with scaled and unscaled wavenumbers and infrared and Raman intensities. The temperature dependent Raman spectrum is reported from room temperature to −100°C. Only three of the five possible conformers can be identified in this spectrum, and there is no evidence of the other two. The energy differences between conformers in the liquid phase were found experimentally to be 132±27, 232±46 and 106±30 cm −1, respectively between the D and C, B and C and D and B conformers. These differences are substantially less than the differences calculated ab initio at the highest level of the theory used, suggesting that energy differences were decreased by large dipole–dipole interactions present in the liquid but not in the gas.

NMR spin–spin coupling constants in water molecule: equilibrium and rovibrational values

Equilibrium 1JOH and 2JHH coupling constants and their respective rovibrational contributions have been calculated for water at the complete active space (CAS) and restricted active space (RAS) multiconfigurational self-consistent field (MCSCF) level using large basis sets. An additive model that considers the effect on the coupling constants of excitation of more than two electrons and of core–electron correlation is used to estimate the coupling constants. The best calculated 1JOHTO and 2JHHTO values (−76.71 and −7.76Hz, respectively) are close to the experimental ones (−80.6 and −7.34Hz). The core–electron correlation effects on 1JOH and 2JHHTO amount, respectively, −1.46 and −0.12Hz. The rovibrational contributions were calculated from a spin–spin coupling surface fitted up to quartic order.

Accuracy of RCC-SD andPT2/CI methods in all-electron and RECP calculations on Pb and Pb2+

Transition energy calculations for low-lying states of Pb and Pb2+ bythe four-component versions of the Fock-space RCC-SD and PT2/CImethods are reported. Contributions of valence and core electroncorrelation are studied in all-electron calculations with theDirac-Coulomb Hamiltonian. The accuracy of our generalized RECP and theRECP of Christiansen and co-workers is tested. The consideration of onlyone- and two-body amplitudes for valence electrons in the RCC method forPb is shown not to be sufficient to reproduce valence excitationswithin 100-300 cm-1. Correcting RCC-SD results by estimatedcontributions of triple and quadruple excitations yields an accuracy ofabout 200 cm-1.

Convergence testing of the analytic representation of an ab initio dipole moment function for water: Improved fitting yields improved intensities

In general, when computing intensities for polyatomics, one has to interpolate the dipole moment function obtained from ab initio calculations. For some high overtones of the water molecule, the computed intensities can be very sensitive to the way in which the interpolation is done. Our previous analytic representation [H. Partridge and D. W. Schwenke, J. Chem. Phys. 106, 4618 (1997)] was not adequate. We show that stable results can be obtained, and these results are in much improved agreement with experiment. We also test the importance of core electron correlation on intensities, and find the effect to be negligible. Of the existing water dipole moment functions in the literature, the present one is the most accurate.

Applications of effective core potentials and density functional theory to the spin states of iron porphyrin.

We investigated the performance of the B3LYP density functional in combination with ab initio effective core potentials (ECPs) that are derived from either Hartree-Fock or Dirac-Fock calculations. The transferability of ab initio ECPs is assessed on the basis of comparison with all-electron density functional calculations. For iron(II) porphyrin in particular, our assessment focused on the relative energetic ordering of five low-lying spin states, 1A1G, 3A1G, 3B2G, 5A2G, and 5B1G, and their properties, including optimized structures, charge distribution, spin density, and vibrational frequencies. Our results show that core electron correlation and core-valence electron correlation do not have significant effects on the relative energetics of the spin states of iron porphyrin. Our calculations suggest that effects of replacing the core electrons with ECPs are less significant than the choice of basis functions. We conclude that ab initio ECPs such as LANL2, RCEP, and MEFIT-R may be combined with the B3LYP density functional theory to provide consistent and accurate results.

The most stable isomers in HC4N and HC6N

The relative stability of the triplet-linear A to the singlet B with a C3-ring in an HC4N molecule was investigated at the density functional level of theory with Becke's three parameters (B3LYP) and the coupled-cluster singles and doubles with triple contributions (CCSD(T)). The various qualities of the correlation-consistent basis sets were used. After the zero-point vibrational energy corrections, the singlet B is by 4.2kcal/mol lower in energy than the triplet A at the present highest level of theory (CCSD(T)/cc-pCVTZ with core electron correlations (CCSD(T)(full)/cc-pCVTZ)). The B3LYP calculations failed to predict the relative stability, however, the optimized geometries even with the cc-pVDZ basis set are in good agreement with the accurate CCSD(T)/cc-pVTZ ones. For the larger HC6N molecule, the CCSD(T)/cc-pVTZ and cc-pCVDZ energy calculations (with and without core electron correlations) on the basis of the B3LYP optimized parameters were performed to predict the relative stabilities of the triplet-linear C to the two singlet-rings (D and E). The triplet C is the most stable isomer, which are by 2.4 and 4.0kcal/mol lower in energy than the singlets D and E, respectively, after the corrections due to the zero-point vibrational energies.

Theoretical investigation of Ca⋅RG, Ca+⋅RG, and Ca2+⋅RG (RG=Ar and Ne) complexes

The ground state structure, harmonic frequency, and dissociation energy for Ca⋅RG, Ca+⋅RG, and Ca2+⋅RG (RG=Ar and Ne) complexes are computed at four theoretical levels [HF, B3LYP, MP2, and MP2(full)] using three different basis sets. The most rigorous method employed was Møller–Plesset second order perturbation with valence plus core electron correlation using 183 basis functions for the calcium–neon complexes and 187 basis functions for the calcium–argon complexes. Correcting the dissociation energies, bond distances, and frequencies for basis set superposition error (BSSE) were done at the most rigorous level of theory by fitting the Morse function to the potential energy curves generated by the counterpoise procedure. At this level of theory, proceeding from the neutral to the doubly charged complexes, the calcium–neon bond distances range from 5.40 to 2.45 Å with dissociation energies (De) from 0.03 to 5.86 kcal/mol. Likewise, the calcium–argon bond distances range from 5.00 to 2.70 Å with dissociation energies from 0.23 to 16.80 kcal/mol as the metal charge increases. Good theoretical agreement is obtained with experimental data when available, while the remaining results can aid in the interpretation of future experiments. In all comparable cases where the calcium–rare gas complexes possess equivalent charge, the argon atom is bound tighter to the metal than the neon atom due to its larger atomic polarizability. An examination of the relationship between dispersion and charge-induced dipole forces is done using these calcium–rare gas complexes.

The equilibrium structure and relative energy of cis- and trans-hydroxysilylene, HSiOH

The equilibrium structure of the cis and trans isomers of hydroxysilylene, HSiOH, has been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double- through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The complete-basis-set limits of the molecular parameters were estimated using various extrapolation techniques. At 0 K, the cis isomer is predicted to be more stable than the trans isomer by merely 0.04±0.03 kcal/mol.

Ab initio calculation of the interaction potential for the krypton dimer: The use of bond function basis sets

The Kr2 interaction potential is studied by ab initio calculations using several large basis sets containing high polarization functions and/or bond functions. It is shown that the addition of bond functions results in a dramatic improvement for the convergence of the calculated interaction energies. At the frozen-core MP4 level, the large atomic basis set such as [9s7p4d3f2g] recovered less than 75% of the experimental well depth. In contrast, the bond function basis set such as [9s7p4d3f]-{3s3p2d1f} produced a well depth of 617 μhartrees, over 99% of the experimental well depth. The frozen-core MP4 calculation appears to overestimate the well depth by about 25 μhartrees as compared to the calculation at the CCSD(T) level. On the other hand, the inclusion of core electron correlation at the MP4 level may contribute 13 μhartrees to the well depth. Beyond the potential minimum, the use of bond functions consistently gives significant improvement in the calculated potential from the highly repulsive wall to the attractive tail region. Final remarks are made about the counterpoise method and the use of bond functions.

One-electron pseudopotential study of NanFn−1 clusters (2⩽n⩽29). I. Electronic and structural properties of the ground state

We introduce a one-electron pseudopotential model to study the structural and electronic properties of excess-electron alkali halide clusters. This model assumes total charge transfer between alkali and halide atoms. This ionic part of the system is described via repulsive and Coulomb potentials. The remaining electrons of the excess metal atoms are treated within an explicit quantal scheme via ion–electron pseudopotentials. Moreover, explicit core-polarization and core-electron correlation contributions are taken into account. This model is used to derive ground state structural, energetics, and electronic properties of one-excess electron NanFn−1 clusters in the range 2⩽n⩽29. We show that the structural characters are closely related with electron localization and we propose a classification into five types, two of them exhibiting rather strong localization namely F-centers and Na-tail structures, the others exhibiting a less bound electron localizing in a surface-state, an edge-state, or on an atom-depleted face of the cluster. Although we observe an energetical predominance of cubiclike structures, hexagonal isomers are seen to appear as stable ones and exhibit similar localization features. The various energy contributions to the stability are examined. All studied NanFn−1 clusters are found stable with respect to fragmentation. The ionization potentials, which are seen to reflect faithfully the localization character, are discussed in details and compared with consistent recent experimental data.

The vibrational–rotational energy levels of silanone

The equilibrium structure and 6-dimensional potential energy surface of silanone, H 2SiO, has been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double- through quintuple-zeta quality. The effects of core-electron correlation on the calculated structural parameters have been investigated. The anharmonic force field has been determined. The vibrational–rotational energy levels of the molecule have then been calculated using variational and perturbational methods. The calculated molecular properties are in good agreement with experimental data.

Calculation of mass polarization for the and states in Li-like ions

Mass polarization arises from a correlation between electronic momenta. In Li-like systems, the effect of core electron correlation dominates. The mass polarization for the and states in B III-Ne VIII is calculated with the use of configuration interaction (CI) wavefunctions. Its dependence on the nuclear charge Z and the atomic states is then studied. The effect of core electron correlation is clearly illustrated with a simple CI wavefunction.

An ab initio study on the equilibrium structure and torsional potential energy function of carbodiimide

The molecular parameters of carbodiimide (HNCNH) have been determined in large-scale ab initio calculations using the coupled-cluster method (CCSD(T)) and the correlation-consistent basis sets of double- through quadruple-zeta quality. The effect of core-electron correlation on the structural parameters was investigated. The equilibrium structural parameters of the molecule were estimated to be r(CN)=1.2242 Å, r(NH)=1.0092 Å, ∠(NCN)=170.62°, ∠(CNH)=117.84°, and ∠(HN…NH)=89.18°. The potential energy barriers for the trans and cis conformers were determined to be nearly equal, with the values 2636 and 2556 cm −1, respectively. The calculated molecular parameters were found to be in good agreement with experiment.

Potential Energy Surface and Vibrational−Rotational Energy Levels of Hydrogen Peroxide

The six-dimensional potential energy surface of hydrogen peroxide, H2O2, has been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with the basis set of quadruple-ζ quality, cc-pVQZ. The effects of core-electron correlation on the calculated structural parameters and the torsional potential energy function have been investigated. The anharmonic quartic force field has been determined. The vibrational−rotational energy levels of the molecule have then been calculated using the variational method. The calculated molecular properties are found to be in good agreement with experimental data.

Calculation of 3s photoemission spectra of 3d transition metal atoms adsorbed on graphite

Abstract The 3 s photoemission spectra of V and Fe adatoms on graphite are calculated within a cluster model. The model takes into account intra-atomic d – d and d –core electron correlation in the transition metal adatom as well as hybridization between transition metal d and graphite π orbitals. The spectra and the magnetic state of the adatom are studied as a function of adatom–surface distance h . When h is increased, the transition metal atom undergoes a low-high spin transition, where the total spin switches from 0 or 1/2 to the Hund's rule value. This transition can be explained by the competition between the d – d exchange splitting and an effective crystal field splitting. In the low spin regime the spectra show essentially one charge transfer satellite, which is quite large for the Fe adatom. In the high spin regime, exchange and charge transfer satellites are mixed leading essentially to a three peak structure. Only at adatom–surface distances much higher than the equilibrium value do the spectra show the simple atomic-like exchange splitting.