[en] A general method for the energy-adjustment of accurate relativistic ab initio pseudopotentials to be used in one-, two- or four-component quantum chemical valence-only calculations of the electronic structure and spectra of heavy-element compounds is discussed. The described pseudopotential scheme is completely based on quantum chemical observables, i.e., all parameters are determined by a least-squares adjustment to total valence energies of a multitude of many-electron states from corresponding atomic all-electron calculations. The presentation will include several examples of theoretical investigations of heavy-element compounds, many of them containing lanthanide or actinide atoms

[en] Nonrelativistic and quasirelativistic energy-adjusted pseudopotentials for fixed 4f subconfigurations of the rare earth elements La through Lu and corresponding optimized valence basis sets have been used in CI(SD) calculations to determine the molecular parameters of energetically low lying superconfigurations of selected lanthanide monoxides, monofluorides and trihalogenides. The experimentally observed trends in dissociation energies and bond lengths of the ground states belonging the calculated superconfigurations are well reproduced. (author). 24 refs,; 3 figs.; 3 tabs

[en] Nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials are presented for element 105 (hahnium, Ha) together with corresponding energy-optimized valence basis sets. The method of energy adjustment of pseudopotentials is extended to a two-component formalism and to multiconfiguration wave functions. The accuracy of the pseudopotential scheme is demonstrated by a comparison of atomic valence-only results to corresponding all-electron data. Atomic multiconfiguration self-consistent field and multireference configuration interaction calculations for M and M⁺ (M = Nb, Ta, Ha) are compared with available experimental data. Corresponding molecular calculations, which included spin-orbit coupling, have been performed for the low-lying states of HaO and are compared to the results from corresponding calculations of the lighter homologs NbO and TaO. 41 refs., 1 fig., 8 tabs

[en] Nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials substituting the 1s--3d core orbitals with corresponding spin--orbit operators for the rare earth elements Ce through Yb have been generated. Excitation and ionization energies from numerical pseudopotential calculations differ by less than 0.1 eV from corresponding numerical all--electron results. The pseudopotentials for Ce have been tested in molecular calculations for the ³Phi ground state of CeO. The derived spectroscopic constants from quasirelativistic pseudopotential CI(SD) calculations including Davidson's correction (R/sub e/ = 1.827 A, D/sub e/ = 6.95 eV, ω/sub e/ = 834 cm^-1) are in good agreement with experimental values (R/sub e/ = 1.820 A, D/sub e/ = 8.19 eV, ω/sub e/ = 862 cm^-1)

[en] We have performed ab initio all-electron and pseudopotential MO calculations on FeCO with varying degrees of sophistication in the basis sets as well as in the calculational procedures. Basis sets with at least double zeta representation for valence orbitals as well as inclusion of correlation corrections in the MO calculations are the minimum requirements in order to arrive at a clear decision about the electronic ground state of FeCO. In contradiction to the suggestion given by Guenzburger et al. we derive from total energy considerations and from comparing experimental and calculated C-O stretching force constants that the ground state is a spin-quintet (⁵Σ^-) and not a spin-triplet. Our calculated quadrupole splitting for MO equilibrium geometry of FeCO ⁵Σ^- deviates by about 25% from the value which was observed by Peden et al. for FeCO in solid noble gas. However, the calculated electronic configuration of iron within FeCO ⁵Σ^- of about 3d^6.44s^0.8 is far from 3d⁶4s², which approximately is expected for FeCO when comparing isomer shifts of FeCO and Fe, both matrix-isolated in solid noble gas, i.e. -0.60 mm/s and -0.75 mm/s, respectively. Thus, at the moment the question remains open to what extent the molecular properties of FeCO are affected by the solid noble gas matrix. (orig.)

[en] A recently proposed computational scheme based on local increments has been applied to the calculation of correlation contributions to the cohesive energy of the CaO crystal. Using ab initio quantum-chemical methods for evaluating individual increments, we obtain ∼80% of the difference between the experimental and Hartree-Fock cohesive energies. Lattice constants corrected for correlation effects deviate by less than 1% from experimental values, in the case of MgO and CaO. copyright 1996 The American Physical Society

[en] Ab initio quantum-chemical methods are not necessarily restricted to molecules and have already been applied to calculate cohesive properties of semiconductors. We extend this method to ionic solids (MgO, CaO, NiO) and calculated cohesive energies and lattice constants. We obtain ∼ 80 % of the correlation contribution to the cohesive energy. Contributions due to the formation of ions are of the same order of magnitude for the cohesive energy as van der Waals-like interactions between the ions. Including correlations, the calculated lattice deviate by less than 1 % from the experimental values. Two main effects arising from correlations are found: the van der Waals-like interaction between the ions reduces the lattice constant whereas intra-atomic correlation of the oxygen ion enforces a larger lattice constant due to the lower level spacing and therefore increasing importance of correlations at a larger lattice constant. First and second ionization potential values of magnesium, calcium, and nickel were calculated. (authors)

[en] The authors present scalar-relativistic energy-consistent Hartree-Fock pseudopotentials for the main-group elements. The pseudopotentials do not exhibit a singularity at the nucleus and are therefore suitable for quantum Monte Carlo (QMC) calculations. They demonstrate their transferability through extensive benchmark calculations of atomic excitation spectra as well as molecular properties. In particular, they compute the vibrational frequencies and binding energies of 26 first- and second-row diatomic molecules using post-Hartree-Fock methods, finding excellent agreement with the corresponding all-electron values. They also show their pseudopotentials give superior accuracy than other existing pseudopotentials constructed specifically for QMC. Finally, valence basis sets of different sizes (VnZ with n=D,T,Q,5 for first and second rows, and n=D,T for third to fifth rows) optimized for our pseudopotentials are also presented

[en] Recently developed quasirelativistic energy-consistent 5f-in-core pseudopotentials as well as core-polarization potentials corresponding to tetravalent actinides have been used to study the molecular structures, ionic metal-ring binding energies, and Mulliken orbital populations of the actinocenes An(C₈H₈)₂ (An = Th-Pu). The results of Hartree-Fock calculations show only small deviations from 5f-in-valence calculations, although the 5f orbitals contribute to the covalent actinide-ring bonding. For thorocene and uranocene, second-order Moller-Plesset perturbation theory, coupled-cluster theory with single and double excitation operators and a perturbative estimate of triple excitations, and density functional theory yield actinide-ring distances which are in good agreement with the experimental values