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5 páginas. ; Time-dependent current–density-functional theory (TDCDFT) provides an, in principle, exact scheme to calculate efficiently response functions for a very broad range of applications. However, the lack of approximations valid for a range of parameters met in experimental conditions has so far delayed its extensive use in inhomogeneous systems. On the other side, in many-body perturbation theory accurate approximations are available, but at a price of a higher computational cost. In the present work, the possibility of combining the advantages of both approaches is exploited. In this way, an exact equation for the exchange-correlation kernel of TDCDFT is obtained, which opens the way for a systematic improvement of the approximations adopted in practical applications. Finally, an approximate kernel for an efficient calculation of spectra of solids and molecular conductances is suggested and its validity is discussed. ; Acknowledge the support from the European Union's (EU's) 7th framework program through the ETSF e-I3 infrastructure project (Grant No. 211956), the Spanish MEC (FIS2007-65702-C02-01), ACI Promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07), and ETORTEK projects. ; Peer reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al. ; We characterize experimentally and theoretically the collective electronic excitations in two prototypical layered transition-metal dichalcogenides, NbSe2 and Cu0.2NbS2. The energy- and momentum-dependent dynamical structure factor was measured by inelastic x-ray scattering (IXS) spectroscopy and simulated by time-dependent density-functional theory. We find good agreement between theory and experiment, provided that Nb semicore states are taken into account together with crystal local-field effects. Both materials have very similar spectra, characterized by two main plasmons at 9 and 23 eV, which we show to both have π+σ character on the basis of a detailed analysis of the band structure. Finally, we discuss the role of the layer anisotropy in the dispersion of these plasmons. ; We acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish grant (2010-21282-C02-01), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), European Commission project CRONOS (Grant No. 280879-2). This research was also supported by a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme, and by Generalidad Valenciana (ISIC Nano program). S.H., K.O.R. and C.J.S. were supported by the Academy of Finland (projects 1256211, 1254065, 1259526) and University of Helsinki Research Funds (project 490076). ; Peer Reviewed