Thank you for your answers. I can break down the problem to the case with no external field:
LATTICE="chain lattice" MODEL="spin" but with only this bondterm: -J*Sx(i)*Sx(j) h=Gamma=D=0 L=6,J=1
dmrg: SWEEPS=4,MAXSTATES=128 dirloop: THERMALIZATION=2000,SWEEPS=20000, EPSILON=0.2,T=0.01
The results for the ground state energy are: Fulldiag: -1.5 Dirloop: -1.0 Dmrg: -1.28
with CONSERVED_QUANTUMNUMBERS=Sz: Fulldiag: -1.0 Dirloop: -1.0 Dmrg: -1.28
and with -J*Sz(i)*Sz(j): Fulldiag: -1.5
I think there is no superposition of bases with Sz_total!=0 in the constraint case.
thanks in advance Armin