Spin Gaps in a Frustrated Heisenberg Model for CaV4O9.

I report results of a density matrix renormalization group (DMRG) study of a model for the two-dimensional spin-gapped system CaV{sub 4}O{sub 9}. This study represents the first time that DMRG has been used to study a two-dimensional system on large lattices, in this case as large as 24{times}11, allowing extrapolation to the thermodynamic limit. I present a substantial improvement to the DMRG algorithms which makes these calculations feasible. {copyright} {ital 1996 The American Physical Society.}