  In this work we have performed a series of simulations of the atmosphere of
GJ~1214b assuming different metallicities using the Met Office Unified Model
(UM). The UM is a general circulation model (GCM) that solves the deep,
non-hydrostatic equations of motion and uses a flexible and accurate radiative
transfer scheme, based on the two-stream and correlated-$k$ approximations, to
calculate the heating rates. In this work we consistently couple a well-tested
Gibbs energy minimisation scheme to solve for the chemical equilibrium
abundances locally in each grid cell for a general set of elemental abundances,
further improving the flexibility and accuracy of the model. As the metallicity
of the atmosphere is increased we find significant changes in the dynamical and
thermal structure, with subsequent implications for the simulated phase curve.
The trends that we find are qualitatively consistent with previous works,
though with quantitative differences. We investigate in detail the effect of
increasing the metallicity by splitting the mechanism into constituents,
involving the mean molecular weight, the heat capacity and the opacities. We
find the opacity effect to be the dominant mechanism in altering the
circulation and thermal structure. This result highlights the importance of
accurately computing the opacities and radiative transfer in 3D GCMs.
