Density-Functional Theory and Excitation Energies

• During the past decade, the local-density (LD) approximation within density-functional (DF) theory has been the most important method for obtaining the electronic properties of realistic systems. The limitations of the method have to a large extent been computational in nature rather than theoretical. The conceptually simple one-particle equations arising in the method have often been too difficult to solve in systems with low symmetry such as amorphous systems or surfaces. However, due to our increased under­standing of these systems the computational techniques are quickly developing, and we forsee an even greater importance of the method in the near future. The theoretical limitations of the method will then become more evident and result in an urgent need for improve­ments beyond the LD approximation. Such a need exists already to­day in many systems such as, e.g., atoms and molecules. The des­cription of correlation effects in these systems is necessary and important and the answers provided by the LD approximations are often too poor to be of practical use. Unfortunately, for a long time, only minor theoretical advances were made since the modern version of DF theory was laid down by Hohenberg, Kohn, and Sham1,2 in the mid sixties.

Subject headings

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

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