Naoum C. Bacalis

Theory for the direct construction of diabatic states and application to the He+22Σ+g spectrum

Abstract We present a theory for the direct construction of correlated wavefunctions representing diabatic states. Attention is given to the proper separation and subsequent computation of state-specific, multiconfigurational, optimized wavefunctions whose main features do not change significantly with geometry and whose mixing causes the breakdown of diabaticity. In the case of diatomics, the theory is implemented via a method which obtains the zeroth-order description as a numerical MCHF function and the remaining “diabatic” correlation as an expansion in terms of numerical diatomic orbitals whose effective charges are optimized from the minimization of the energy. Transparent as well as accurate wavefunctions emerge. Application to the He + 2 2 Σ + g valence and Rydberg states has produced results in agreement with those derived from knowledge of adiabatic curves and of d/d R matrix elements, obtained from extensive LCAO-MO configuration-interaction calculations.

Excited molecules and clusters in solid media. Hydrogen and tetrahydrogen in ionic crystals

Abstract We present accurate results from full CI calculations on ground and excited states of H 2 and (H 2 ) 2 embedded in AgF and RbI solids. It is found that the effect of these crystals on the spectra and on the energy surface characteristics is considerable. This finding suggests that, with a suitable selection of solid media, it may become possible to manipulate substantially the electronic spectroscopy and the energy storage and dissipation of certain classes of molecules and clusters.

Exponential tails in the density of states

The exponential tails in the density of states in amorphous tetrahedral films, chalcogenide glasses, and ionic crystals are associated with local states bound in single potential “wells”. The exponential distribution is due (a) to the Gaussian distribution of the depths of the single potential “wells”, and (b) to an approximately linear relation between the binding energy and the square of the depth of the potential well. This simple picture accounts for the qualitative features of the Urbach tails and it is consistent with quantitative data in a-Si:H, As2Se3, and in ionic crystals. Various levels of refinement of this simple picture have been worked out by us and our collaborators. These refinements provide quantitative improvement in the Urbach region, and extend the validity of our treatment to the Halperin-Lax and Tauc regions as well as the extremely deep states.

Toward the understanding of He2− excited states

Using a recently developed state-specific method for the calculation of wavefunctions of diatomic molecules (N.C. Bacalis, Y. Komninos and C.A. Nicolaides, Phys. Rev. A 45 (1992) 2701), we calculated the potential energy curve for a localized wavefunction of a 4Φg(1σg21σu2πu1δg) state in He2−, including including electron correlation. This state is a Feshbach resonance, lying below its parent He2 3Πg(1σg21σu2πu) and 3Δu(1σg2 1σu1δg) excited states and autoionizing into the continuum of He2 3Πg(1σg21σu1πu) by the two-orbital rearrangement 2πu1δg → 1πuϵδg or 1πuϵγg (in σ, π, δ, ϕ, γ 3. diatomic notation). From our state-specific numerical Hartree-Fock calculations it can be concluded that the localized solution for the 4Ig(1σg21σu1δg1ϕu) state obtained by L. Adamowicz and T.P. Pluta (Chem. Phys. Letters 179 (1991) 517), should five rise to a shape resonance above the lowest He2 3Δu(1σg21σu1δg) state.

Transferable tight-binding parameters for ferromagnetic and paramagnetic iron

We construct transferable tight-binding (TB) parameters for ferromagnetic and paramagnetic iron by fitting the total energy and the electronic band structure to three prototype crystal structures of Fe (BCC, both ferromagnetic and paramagnetic; and FCC, paramagnetic only) calculated by the general-potential linearized augmented plane wave (LAPW) method. We use these TB parameters to calculate the total energy and other properties of Fe in various other crystal structures, which we compare with independent LAPW results. The agreement between LAPW and TB results is very good, suggesting a realistic parametric physical description in the tight-binding approximation for any structure of Fe.

Applications of the NRL tight-binding method to magnetic systems

The NRL developed tight-binding method has been very successful in describing the properties of nonmagnetic elemental metals and semiconductors with accuracy comparable to first-principles methods. In this article we discuss extensions of the method to magnetic systems. We first show that the method correctly predicts equilibrium ground state structures, elastic constants, and phonon frequencies in ferromagnetic iron. We also show how the magnetic calculations can be extended to noncollinear systems, focusing on the electronic behavior of iron.

Calculation of the electron momentum density and compton scattering measurements for nickel

The (100), (110) and (111) directional Compton profiles of single crystals of nickel have been measured using 59.54 keV 241Am gamma -rays at a 170 degrees scattering angle and a well defined scattering vector. The electron momentum distribution and Compton profiles in nickel have also been calculated on the basis of a self-consistent augmented-plane-wave method within the local-density approximation including a correlation correction term. A quantitative comparison between experimental and theoretical directional Compton profiles shows good agreement and reveals some accurate information about the Fermi surface. Excitations due to nonlocal correlation effects from contributing parts to noncontributing parts of the energy bands are very clearly seen along the (100), (110) and (111) directions. The calculation also reproduces quite well the main features of the observed anisotropies, typically within the experimental error margins, although it slightly overestimates their magnitude.

Existence of He2- negative ions with two remote electrons in antibonding orbitals

Calculations of low-lying quartet states of He2 - are reported, in which a positively charged core He2 + holds two remote electrons in antibonding orbitals, i.e. spending most time either side of and away from the central nodal plane, of similarly large RMS extent. A previously reported 4 u state is reinterpreted and shown to be a Rydberg state. Further, a 4 u state and a 4 g , the lowest state of this category, are reported. The calculation, performed in prolate spheroidal coordinates, is variational, utilizing one-electron diatomic-molecule-type orbitals, in full configuration interaction.

Variational predictability of diabatic, adiabatic or impossible diatomic states

The use of appropriately screened one-electron diatomic orbitals (OEDOs) in a variational configuration interaction (CI) of ground or excited diatomic states guarantees the nature of the desired state and speeds up the CI convergence. The method can predict, within the variational OEDOs, the possibility or impossibility of a two-centre state and can simultaneously compute both diabatic and adiabatic states. As an application, : is shown to be impossible, and four previously unreported excited states of are calculated.

Calculation of the Compton profiles of vanadium, niobium and their dihydrides

We have calculated the Compton profiles of V, Nb, VH2 and NbH2 using the self-consistent augmented plane wave (APW) method within the local-density approximation of Hedin-Lundqvist. The results are compared with other theoretical works and available experiments. In going from the pure metals to the metal dihydrides we observe significant changes in the directional Compton profiles due to their different band structures.