  Theoretical differences in the growth of structure offer the possibility that
we might distinguish between modified gravity theories of dark energy and
\LambdaCDM. A significant impediment to applying current and prospective large
scale galaxy and weak lensing surveys to this problem is that, while the mildly
nonlinear regime is important, there is a lack of numerical simulations of
nonlinear growth in modified gravity theories. A major question exists as to
whether existing analytical fits, created using simulations of standard
gravity, can be confidently applied. In this paper we address this, presenting
results of N-body simulations of a variety of models where gravity is altered
including the Dvali, Gabadadze and Porrati model. We consider modifications
that alter the Poisson equation and also consider the presence of anisotropic
shear stress that alters how particles respond to the gravitational potential
gradient. We establish how well analytical fits of the matter power spectrum by
Peacock and Dodds and Smith et al. are able to predict the nonlinear growth
found in the simulations from z=50 up to today, and also consider implications
for the weak lensing convergence power spectrum. We find that the analytical
fits provide good agreement with the simulations, being within 1\sigma of the
simulation results for cases with and without anisotropic stress and for
scale-dependent and independent modifications of the Poisson equation. No
strong preference for either analytical fit is found.
