  We present asevolution, a cosmological N-body code developed based on
gevolution, which consistently solves for the (a)symmetron scalar field and
metric potentials within the weak-field approximation. In asevolution, the
scalar field is dynamic and can form non-linear structures. A cubic term is
added in the symmetron potential to make the symmetry-broken vacuum expectation
values different, which is motivated by observational tensions in the late-time
universe. To study the effects of the scalar field dynamics, we also implement
a constraint solver making use of the quasi-static approximation, and provide
options for evaluating the background evolution, including using the full
energy density averaged over the simulation box within the Friedmann equation.
The asevolution code is validated by comparison with the Newtonian N-body code
ISIS that makes use of the quasi-static approximation. There is found a very
small effect of including relativistic and weak-field corrections in our small
test simulations; it is seen that for small masses, the field is dynamic and
can not be accurately solved for using the quasi-static approximation; and we
observe the formation of unstable domain walls and demonstrate a useful way to
identify them within the code. A first consideration indicates that the domain
walls are more unstable in the asymmetron scenario.
