I feel very honored that I have been asked to write a Foreword to this
book. The subject of the book - "Coupled cluster theory" - has been
around for about half a century. The basic theory and explicit equations
for closed-shell ground states were formulated before 1970. At the
beginning of the seventies the rst ab initio calcu- tion were carried
out. At that time speed and memory of computers were very limited
compared to today's standards. Moreover, the size of one-electron bases
employed was small, so that it was only possible to achieve an
orientation in methodical aspects rather than to generate new signi cant
results. Extensive use of the coupled-cluster method started at the
beginning of the eighties. With the help of more powerful computers the
results of coupled-cluster approaches started to yield more and more
interesting results of relevance to the interpretation of experimental
data. New ideas in methodology kept appearing and computer codes became
more and more ef cient. This exciting situation continues to this very
day. Remarkably enough, even the - quired equations can now be generated
by a computer with the help of symbolic languages. The size of this
monograph and the rich variety of articles it contains attests to the
usefulness and viability of the couple-cluster formalism for the h-
dling of many-electron correlation effects. This represents a vivid
testimony of a tremendous work that has been accomplished in
coupled-cluster methodology and its exploitation.
