  The phase diagram of isotropically expanded graphene cannot be correctly
predicted by ignoring either electron correlations, or mobile carbons, or the
effect of applied stress, as was done so far. We calculate the ground state
enthalpy (not just energy) of strained graphene by an accurate off-lattice
Quantum Monte Carlo (QMC) correlated ansatz of great variational flexibility.
Following undistorted semimetallic graphene (SEM) at low strain,
multi-determinant Heitler-London correlations stabilize between $\simeq$8.5%
and $\simeq$15% strain an insulating Kekule-like dimerized (DIM) state. Closer
to a crystallized resonating-valence bond than to a Peierls state, the DIM
state prevails over the competing antiferromagnetic insulating (AFI) state
favored by density-functional calculations which we conduct in parallel. The
DIM stressed graphene insulator, whose gap is predicted to grow in excess of 1
eV before failure near 15% strain, is topological in nature, implying under
certain conditions 1D metallic interface states lying in the bulk energy gap.

Materials Science;Mesoscale and Nanoscale Physics;Strongly Correlated Electrons