  We apply density functional theory band structure calculations and quantum
Monte Carlo simulations to investigate the Bose-Einstein condensation in the
spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the
one-dimensional nature of this compound, we present a quasi-two-dimensional
model of spin dimers with ferromagnetic and antiferromagnetic interdimer
couplings. Our model is well justified microscopically and provides a
consistent description of the experimental data on the magnetic susceptibility,
high-field magnetization, and field vs. temperature phase diagram. The
Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9
is largely governed by intralayer interactions, whereas weak interlayer
couplings have a moderate effect on the ordering temperature. The proposed
computational approach is an efficient tool to analyze and predict high-field
properties of quantum magnets.
