  Instability of perovskite photovoltaics is still a topic which is currently
under intense debate, especially the role of water environment. Unraveling the
mechanism of this instability is urgent to enable practical application of
perovskite solar cells. Here, ab initio metadynamics is employed to investigate
the initial phase of a dissolution process of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$)
in explicit water. It is found that the initial dissolution of MAPbI$_3$ is a
complex multi-step process triggered by the departure of I$^-$ ion from the
CH$_3$NH$_3$I-terminated surface. Reconstruction of the free energy landscape
indicates a low energy barrier for water dissolution of MAPbI$_3$. In addition,
we propose a two-step thermodynamic cycle for MAPbI$_3$ dissolution in water at
a finite concentration that renders a spontaneity of the dissolution process.
The low energy barrier for the initial dissolution step and the spontaneous
nature of MAPbI$_3$ dissolution in water explain why the water immediately
destroys pristine MAPbI$_3$. The dissolution thermodynamics of all-inorganic
CsPbI$_3$ perovskite is also analyzed for comparison. Hydration enthalpies and
entropies of aqueous ions play an important role for the dissolution process.
Our findings provide a comprehensive understanding to the current debate on
water instability of MAPbI$_3$.
