These two volumes collect thirty-eight selected papers from the scientific contributions presented at the Fourth European Workshop on Quantum Systems in Chemistry and Physics (QSCP-IV), held in Marly-le-Roi (France) in April 22-27, 1999, A total ofone hundred and fifteen scientists attended the workshop, 99 from Europe and 16 from the rest ofthe world. They discussed the state of the art, new trends, and future evolution of the methods and applications. The workshop was held in the old town of Marly-le-Roi, which lies to the West of Paris between the historic centres of Saint-Germain-en-Laye and Versailles. Participants were housed at the National Youth Institute, where over sixty lectures were given by l- ding members ofthe scientific community; in addition, over sixty posters were presented in two very animated sessions. We are grateful to the oral speakers and to the poster p- senters for making the workshop such an stimulating experience. The social programme was also memorable - and notjust for the closing banquet, which was held at the French Senate House. We are sure that participants will long remember their visit to the 'Musée des Antiquités Nationales': created by Napoleon III at the birthplace ofLouis XIV, this museum boasts one ofthe world finest collections ofarcheological artifacts. The Marly-le-Roi workshop followed the format established at the three previous meetings, organized by Prof.

Preface. Part I: Density Matrices and Density Functionals. Are exact Kohn-Sham potentials equivalent to local functions? R.K. Nesbet, R. Colle. A theory of exact exchange relations for a single excited state; ÿ. Nagy. Correlation energy contributions from low-lying states to density functionals in the KLI approximation; C. Gutle, et al. Orbital local-scaling transformation approach to fermionic systems in the ground state; Ya.I. Delchev, et al. Reduced density matrix treatment of spin-orbit interaction terms in many-electron systems; R.L. Pavlov, et al. Part II: Electron Correlation Treatments. Many-electron Sturmians applied to atoms and ions in strong external fields; J. Avery, C. Coletti. An implementation of the configuration-selecting multireference configuration-interaction method on massively parallel architectures; P. Stampfuß, W. Wenzel. Comments on the basis sets used in recent studies of electron correlation in small molecules; S. Wilson, et al. Part III: Relativistic Formulations and Effects. Relativistic quantum mechanics of atoms and molecules; H.M. Quiney. Variational principle in the Dirac theory: spurious solutions, unexpected extrema and other traps; M. Stanke, J. Karwowski. Relativistic multireference many-body perturbation theory; M.J. Vilkas, et al. Relativistic valence-bond theory and its application to metastable Xe2; S. Kotochigova, et al. Relativistic quantum chemistry of superheavy transactinide elements; G.L. Malli. Part IV: Valence Theory. The nature of binding in HRgY compounds (Rg = Ar, Kr, Xe; Y = F, Cl) based on the topological analysis of the electron localisation function (ELF); S. Berski, et al. Symmetry separated (sigma+pi vs bent bond (Omega) models of first-row transition-metal methylene cations; F. Ogliaro, et al. Hartree-Fock study of hydrogen-bonded systems in the absence of basis-set superposition error: the nucleic-acid base pairs; A. Famulari, et al. Proton transfer and non-dynamical correlation energy in model molecular systems; H. Chojnacki. Part V: Nuclear Motion. Large-amplitude motions in electronically excited states: a study of the S1 excited state of formic acid; L.M. Beaty-Travis, et al. Ab-Initio harmonic analysis of large-amplitude motions in ethanol dimers; M.L. Senent, et al. Vibrational first hyperpolarizability of methane and its fluorinated analogs; O. Quinet, B. Champagne. Staggering effects in nuclear and molecular spectra; D. Bonatsos, et al. Contents of Volume 2. Combined Index to Volumes 1 and 2.