  Using both exact diagonalizations and diagonalizations in a subset of
short-range valence bond singlets, we address the nature of the groundstate of
the Heisenberg spin-1/2 antiferromagnet on the square lattice with competing
next-nearest and next-next-nearest neighbor antiferromagnetic couplings
(J1-J2-J3 model). A detailed comparison of the two approaches reveals a region
along the line (J2+J3)/J1=1/2, where the description in terms of
nearest-neighbor singlet coverings is excellent, therefore providing evidence
for a magnetically disordered region. Furthermore a careful analysis of
dimer-dimer correlation functions, dimer structure factors and
plaquette-plaquette correlation functions provides striking evidence for the
presence of a plaquette valence bond solid order in part of the magnetically
disordered region.
