A simple coupled-cluster-type approximation is introduced, which gener
alizes in momentum (k) space a method recently presented in the litera
ture for real space, and tested on the exactly solvable XXZ anisotropi
c quantum antiferromagnetic spin-1/2 chain. We implement our approxima
tion in k space on the fermionized version of this model, and obtain i
mproved results, with remarkable quantitative and qualitative features
.