We report ab initio calculations for the interface energetics of a weakly adsorbed organic molecule on a metal surface, which serves as a model interface relevant for organic electronics. The studied thiophene ring is found to be physisorbed on the Cu(110) surface with an adsorption energy of -0.50 eV. Nonlocal correlations, i.e., van der Waals interactions, are solely responsible for the binding in this weakly interacting system, and the choice of the proper exchange-correlation function is crucially important. The adsorption of thiophene lowers the metal work function due to the formation of surface dipoles while no sizable charge transfer is found.

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The crystal structures of Zn and Cd deviate from an ideal hexagonal close packing by a significantly increased c / a ratio. In order to investigate the electronic reason for this deviation, especially with regard to the nearly ideal hcp element Mg, Hartree-Fock and density functional theory calculations were performed, employing various functionals within the local density or the generalized gradient approximations as well as hybrid functionals. The cohesive energy, lattice constants optimized with respect to the energy and elastic constants were computed. The role of electronic correlation in consideration of the filled d-shell is emphasized, postulating different intra-and inter-layer interactions, both in Zn and Cd. On the potential energy surface in the space of varying lattice constants, a path is explored that corresponds to a uniaxial compression along the c axis. In contrast to Mg, the potential energy surface of Zn and Cd is very flat along this path, and an electronic topological transition occurs, leading to a Mg-like band structure.

In a recently performed experiment, ͓Korovyanko et al.Phys. Rev. B 67, 035114 ͑2003͔͒ have measured the photoinduced absorption ͑PA͒ spectrum of phenyl-disubstituted polyacetylenes ͑PDPA's͒ from the 1B u and 2A g excited states. In the 1B u PA spectrum they identified two main features, namely, PA1 and PA2, while in the 2A g spectrum they identified only one feature called PA g . In this paper we present a theoretical study of the 1B u and 2A g PA spectra of oligo-PDPA's using the correlated-electron Pariser-Parr-Pople model and various configuration interaction methodologies. We compare the calculated spectra with the experiments, as well as with the calculated spectra of polyenes of the same conjugation lengths. The calculated spectra are in good agreement with the experiments. Based upon our calculations, we identify PA1 as the mA g state and PA g as the nB u state of the polymer. Regarding the PA2 feature, we present our speculations. Additionally, it is argued that the nature of the excited states contributing to the 2A g -PA spectra of oligo-PDPA's is qualitatively different from those contributing to the spectra of polyenes.

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