  We study a generalized Kane-Mele-Hubbard model with third-neighbor hopping,
an interacting two-dimensional model with a topological phase transition as a
function of third-neighbor hopping, by means of the determinant projector
Quantum Monte Carlo (QMC) method. This technique is essentially numerically
exact on models without a fermion sign problem, such as the one we consider. We
determine the interaction-dependence of the Z2 topological insulator/trivial
insulator phase boundary by calculating the Z2 invariants directly from the
single-particle Green's function. The interactions push the phase boundary to
larger values of third-neighbor hopping, thus stabilizing the topological
phase. The observation of boundary shifting entirely stems from quantum
{\deg}uctuations. We also identify qualitative features of the single-particle
Green's function which are computationally useful in numerical searches for
topological phase transitions without the need to compute the full topological
invariant.
