We present analytical and numerical calculations for some exited states of
the frustrated J1-J2 spin-1/2 Heisenberg model for linear chains and square
lattices. We consider the lowest eigenstates in the subspaces determined by the
eigenvalue M of the spin operator S_total^z. Because of the reduced number of
Ising basis states in the subspaces with higher M we are able to diagonalize
systems with up to N=144 spins. We find evidence that the Marshall-Peierls sign
rule survives for a relatively large frustration parameter J2.Comment: 7 pages, LaTeX, 4 eps figures, to appear in Physica