Cluster expansion of the wavefunction: Ionizations and some low-energy excitations of naphthalene

Ionizations and some lower singlet valence and Rydberg excitations of naphthalene are studied by the SAC/SAC CI method with 100 (44 π + 56 σ) active MOs. A systematic assignment of the ionization spectra is given. In particular, assignments are given for the peaks in the 13–20 eV region. A large σ-electron correlation effect is found for the valence excited B_1u state, together with a large effect of the d_π polarization function. The controversy concerning the assignment of the Rydberg nd_π orbitals is resolved: Robin's assignment of the 3d_π orbital at 6.73 eV above the ground state is supported.     

Cluster expansion of the wavefunction

Numerical examples illustrate the SAC and SAC-CI results for correlatioin energies compared to those from full CI calculations of Bauschlicher and Taylor.     

Vibrational lineshapes of adsorbates: the role of substrate electronic excitations

The vibrational spectrum of an atom or molecule adsorbed on a metallic surface is influenced by the dynamic interaction between the localized oscillator and the continuum of electron-hole pair excitations in the solid. Features of an exactly soluble model for this interacting system are presented, the local spectral function is obtained in terms of calculable level shift and broadening functions, and some prototypical numerical results are displayed and discussed.     

Free-energy based pair-additive potentials for bulk Ni-Al systems: Application to study Ni-Al reactive alloying

We present new numerical pair-additive Al, Ni, and Al-Ni potentials by force-matching (FM) ionic force and virial data from single (bulk liquid) phase ab initio molecular dynamics (MD) simulations using the Born-Oppenheimer method. The potentials are represented by piece-wise functions (splines) and, therefore, are not constrained to a particular choice of analytical functional form. The FM method with virial constraint naturally yields a potential which maps out the ionic free-energy surface of the reference ensemble. To further improve the free energetics of the FM ensemble, the FM procedure is modified to bias the potentials to reproduce the experimental melting temperatures of the reference (FCC-Al, FCC-Ni, B2-NiAl) phases, the only macroscopic data included in the fitting set. The performance of the resultant potentials in simulating bulk metallic phases is then evaluated. The new model is applied to perform MD simulations of self-propagating exothermic reaction in Ni-Al bilayers at P = 0-5 GPa initiated at T = 1300 K. Consistent with experimental observations, the new model describes realistically a sequence of peritectic phase transformations throughout the reaction and at a realistic rate. The reaction proceeds through interlayer diffusion of Al and Ni atoms at the interface with formation of B2-NiAl in the Al melt. Such material responses have, in the past, been proven to be difficult to observe with then-existing potentials.     

Spectrum of electronic excitations due to the adsorption of atoms on metal surfaces

The time-dependent, mean-field Newns-Anderson model for a spin-polarized adsorbate approaching a metallic surface is solved in the wide-band limit. Equations for the time evolution of the electronic structure of the adsorbate-metal system are derived and the spectrum of electronic excitations is found. The behavior of the model is demonstrated for a set of physically reasonable parameters.     

Quantum Monte Carlo for electronic excitations of free-base porphyrin

Accurate calculations of allowed and nonallowed transitions in porphyrin are reported. Using the quantum Monte Carlo method in the diffusion Monte Carlo variant, the vertical transition between the ground state singlet and the second excited state singlet as well as the adiabatic transition between the ground state and the lowest triplet state have been computed for this 162-electron system. The present theoretical results are compared to experiment and to results of other theoretical methods. The diffusion Monte Carlo energy differences are found to be in excellent agreement with experiment.     

Cluster expansion of the wavefunction. Pseduo-orbital theory applied to spin correlation

We have discussed the utility of the cluster expansion of the wavefunction and proposed a pseudo-orbital theory which constitutes a generalization of the orbital-theoretic idea. It has been applied to the calculation of spin densities of the first-row atoms. The results are encouraging in comparison with the experimental values.     

Cage motions induced by electronic and vibrational excitations: Cl2 in Ar

Femtosecond dynamics of molecular vibrations as well as cage motions in the B<--X transition of Cl2 in solid Ar have been investigated. We observed molecular vibrational wave-packet motion in experimental pump-probe spectra and an additional oscillation with a 500 fs period which is assigned to the zone-boundary phonon of the Ar crystal. The cage motion is impulsively driven by the B<--X transition due to the expansion of the electronic cloud of the chromophore. To clarify the underlying mechanism, we performed simulations based on the diatomics-in-molecules method which takes into account the different shapes of the Cl2 electronic wave function in the B and X states as well as the anisotropic interaction with the matrix. The simulation results show that Ar atom motion in the (100) plane is initiated by the electronic transition and that only those Ar atoms oscillate coherently with an approximately 500 fs period which are essentially decoupled from the molecular vibration. Their phase and time evolution are in good agreement with the experimentally observed oscillation, supporting the assignment as a displacive excitation of coherent phonons.     

Chromophores in spectroscopy: Ab initio studies of localized descriptions of molecular electronic excitations

We review a computationally efficient approach, based jointly on the Random Phase Approximation (RPA) and on localized molecular orbitals, for calculating and analyzing electronic excitations in terms of the nature of the chromophore and its interaction with its molecular surroundings. The method is applied to two typical chromophoric systems using ab initio extended-basis calculations: the non-conjugated but electronically coupled ethylenic double bonds in norbornadiene (NBD, bicyclo[2.2.1]hepta-2,5-diene) and the chirally perturbed carbonyl chromophore inequatorial 4-methyladamantanone (EMAO). The analyses are a posteriori in nature but provide insights into the spectroscopic properties of medium-sized molecules.     

Cluster expansion of the wavefunction. Excited states

A method for excited states is given on the basis of the symmetry-adapted-cluster (SAC) expansion method. It is based on the fact that the SAC expansion method gives incidentally a set of excited functions which satisfies the Brillouin theorem with the ground state.     

Cluster expansion for the transport properties of suspensions

The cluster expansion of the transport coefficients of suspensions is discussed. As an exmaple, the effective dielectic constant of a suspension of polarizable spheres is considered. The absolute convergence of the cluster integrals is studied . We show how to derive the Clausius-Mossotti formula from the cluster expansion and how to obtain corrections to that formula. Analogous conclusions hold for the viscosity of a fluid suspension or the elastic constants of a solid suspension.     

Cluster configuration method: application to the spectra of π-electron systems

A simplified method to calculate the excitation spectra of π-electron systems is proposed. The method is based on the assumption that a “cluster” approximation can be applied to excited states. It is demonstrated that, for the low-lying triplet and optically allowed states of butadiene, hexatriene and benzene, the method yields good agreement with complete CI calculations in the Pariser–Pan–Pople approximation.     

Dynamics of surface-localised electronic excitations studied with the scanning tunnelling microscope

The decay rates of electron and hole excitations at metal surfaces as determined by a scanning tunnelling microscope are presented and discussed. Surface-localised electron states as diverse as Shockley-type surface states and quantum well states confined to ultrathin alkali metal adsorption layers are covered. Recent developments in the analysis of the experimental procedures that are used to determine decay rates with the scanning tunnelling microscope, namely the analysis of line shapes and the spatial decay of standing wave patterns, are discussed.     

Density functional theory of the collective electronic excitations in NanKn clusters

The collective electronic response of Na_nK_n clusters has been studied for some model structures. In their low-temperature lowest-energy structure, those clusters have all the K atoms on the surface. The collective oscillation frequencies for clusters with the K atoms segregated to the surface are red-shifted with respect to the corresponding frequencies for isomers with a very similar underlying skeleton but with the Na atoms segregated to the surface. The collective frequency varies smoothly with respect to the degree of relative segregation. These results may be useful in the analysis of the collective response of large alloy clusters and microcrystals. © 1995 John Wiley & Sons, Inc.