PHASE-DIAGRAM OF THE SPIN-ONE HEISENBERG-SYSTEM WITH DIMERIZATION ANDFRUSTRATION

We use the density matrix renormalization group method to study the gr oundstate ''phase'' diagram and some low-energy properties of an antif erromagnetic spin-1 chain with a next-nearest-neighbour exchange J(2) and an alternation delta of the nearest-neighbour exchanges. We find a line running from a gapless point at (J(2), delta) = (0, 0.25 +/- 0.0 1) up to a ''gapless'' point at (0.73 +/- 0.005, 0) such that the open -chain ground state is fourfold degenerate below the line and is uniqu e above it. A disorder line 2J(2) + delta = 1 runs from delta = 1 to a bout delta = 0.136. To the left of this line, the peak in the structur e factor S(q) is at pi, while to the right of the line, it is at less than pi.