The well-known expression for the total energy in terms of the single-particle many-body Green's function is analyzed in detail. In particular the relation between the nth order M011er-Plesset energy and the energy calculated from a Green's function generated by the nth order self-energy is investigated. It is shown how the nth order M011er-Plesset energy can be expressed in terms of the Green's function. The H2 molecule is studied in a minimal basis to serve as a model in which exact results can be easily obtained. Numerical calculations are performed for H 2 , He, Be, LiH, Ne, HF, H 2 0, NH 3 , and CH 4 and the results are analyzed in detail.

We have combined photoelectron spectroscopy (PES) and very-low-energy electron diffraction (VLEED) to study the electronic band structure E͑k͒ of a material with complicated unoccupied upper bands, which are incompatible with the free-electron approximation. Using VLEED, we have experimentally determined these bands, and accordingly optimized the PES experiment. PES band mapping using the VLEED upper bands enabled the first consistent resolution of the lower occupied bands, in particular, the perpendicular dispersion E͑k Ќ ͒, for a layered material. The combination of PES and VLEED is a powerful method to resolve even very complicated E͑k͒ absolutely.[S0031-9007(97)

The valence bands of the layered compound VSe 2 and the related intercalation compounds Na x VSe 2 , K x VSe 2 , and Cs x VSe 2 have been investigated by means of angle-resolved photoelectron spectroscopy, and compared to self-consistent linear augmented plane-wave ͑LAPW͒ band calculations. The intercalation compounds were prepared in situ by deposition of Na, K, and Cs on VSe 2 cleavage surfaces. The intercalation was monitored by core-level spectroscopy, and although K was found to intercalate more slowly than Na and Cs, estimated alkali concentrations of xϭ0.2-0.3 were reached for all three alkali metals. Additional depositions mainly seemed to increase the intercalation depth. Good agreement between LAPW calculations and valenceband spectra was found, in particular for the dispersion along the layers. Normal-emission spectra, obtained at different photon energies, indicated vanishing perpendicular dispersion, but in spectra measured under variation of the emission angle some band-edge signatures were seen, which suggests that some perpendicular dispersion remains, in accordance with the LAPW calculations. The lack of dispersion in the normal-emission spectra could be due to intercalation induced structural transformations, leading to stacking disorder. Also correlation effects may contribute. The rigid-band model is found inadequate, except as a crude approximation, for describing the changes during the initial phase of intercalation. It might be used to describe the continued intercalation, however, under condition that the intercalation modified bands are used. The need for studies that probe both electronic and crystallographic structure ͑including defects͒ is stressed. ͓S0163-1829͑98͒08536-1͔

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Angle-resolved photoemission spectroscopy has been employed to study in situ intercalation of the layered compound VSe2 with Cs. The results show that the valence band structure of VSe2, which initially is of 3D character, is transformed to become essentially 2D upon Cs intercalation. The changes, which go far beyond the rigid-band model, are successfully reproduced by self-consistent linear-augmentedplane-wave calculations. This is the first report of a UHV compatible system where the dimensionality of the electronic structure can be controlled in situ.