Can the eigenstates of a many-body Hamiltonian be represented exactly using a general two-body cluster expansion?
It is proved that a recent conjecture that the exact ground-state wave function of an arbitrary many-fermion system with one- and two-body interactions may be represented by an exponential cluster expansion employing finite two-body operators, starting from any reference function sufficiently close to the exact eigenfunction, is not valid. We show that the space of initial reference functions which lead to the exact ground state is of dimension equal to the number of two-body operators. If the dimension of the multiparticle space is greater than the number of two-body operators, then the space of good reference functions is of measure zero in it.
Ronen
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School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel. sronen@post.tau.ac.il