  Electronic structure calculations for solids based on many-electron
wavefunctions have been hampered by the argument that for large electron
numbers wavefunctions are not a legitimate scientific concept, because they
face an exponential wall problem. We show that this problem can be avoided by
using cumulant techniques in formulating wavefunctions. Therefore calculations
for solids based on many-electron functions are possible and useful. This
includes also systems with strongly correlated electrons.
