  Thermal DMRG is investigated with emphasis of employability in molecular
magnetism studies. To this end magnetic observables at finite temperature are
evaluated for two one-dimensional quantum spin systems: a Heisenberg chain with
nearest-neighbor antiferromagnetic interaction and a frustrated sawtooth
(delta) chain. It is found that thermal DMRG indeed accurately approximates
magnetic observables for the chain as well as for the sawtooth chain, but in
the latter case only for sufficiently high temperatures. We speculate that the
reason is due to the peculiar structure of the low-energy spectrum of the
sawtooth chain induced by frustration.
