The optical spectrum of water is not well understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation, which is contrary to the behavior expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlations, finding that condensation leads to delocalization of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment.

We present equilibrium geometries, vibrational modes, dipole moments, ionization energies, electron affinities, and optical absorption spectra of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient-corrected density-functional theory (DFT-GGA) calculations using ultrasoft pseudopotentials and a plane-wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.

The structural and electronic properties of the quasi-one-dimensional In=Si111 surface system are calculated from first principles. It is found that the symmetry lowering of the In chains is energetically favorable, provided neighboring nanowires are correlated, giving rise to a doubling of the surface unit cell both along and perpendicular to the chain direction. The recently suggested formation of hexagons within the In nanowires [C. González, F. Flores, and J. Ortega, Phys. Rev. Lett. 96, 136101 (2006)]-in clear contrast to the trimer formation proposed earlier -drastically modifies the electron transport along the In chains, in agreement with experiment.

Based on the ab initio density functional theory we study the influence of many-body effects on the quasiparticle ͑QP͒ band structures and optical absorption spectra of highly ionic crystals. Quasiparticle shifts and electron-hole interaction are studied within the GW approximation. In addition to the electronic screening the effect of the lattice polarizability is discussed in detail. Substantial effects are observed for QP bands of AlN and NaCl that have large polaron constants of 1-2. The effect of electronic and lattice polarization on the optical spectra is discussed in terms of dynamical screening and vertex corrections. The results are critically discussed in the light of experimental data available. We find that measured peak positions can be reproduced without lattice polarizability in the screening of the electron-hole interaction and a reduced lattice contribution to the QP shifts.

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