  We define the Anderson hamiltonian on the two dimensional torus $\mathbb
R^2/\mathbb Z^2$. This operator is formally defined as $\mathscr H:= -\Delta +
\xi$ where $\Delta$ is the Laplacian operator and where $\xi$ belongs to a
general class of singular potential which includes the Gaussian white noise
distribution. We use the notion of paracontrolled distribution as introduced by
Gubinelli, Imkeller and Perkowski in [14]. We are able to define the
Schr\"odinger operator $\mathscr H$ as an unbounded self-adjoint operator on
$L^2(\mathbb T^2)$ and we prove that its real spectrum is discrete with no
accumulation points for a general class of singular potential $\xi$. We also
establish that the spectrum is a continuous function of a sort of enhancement
$\Xi(\xi)$ of the potential $\xi$. As an application, we prove that a correctly
renormalized smooth approximations $\mathscr H_\varepsilon:= -\Delta +
\xi_\varepsilon+c_\varepsilon$ (where $\xi_\varepsilon$ is a smooth
mollification of the Gaussian white noise $\xi$ and $c_\varepsilon$ an explicit
diverging renormalization constant) converge in the sense of the resolvent
towards the singular operator $\mathscr H$. In the case of a Gaussian white
noise $\xi$, we obtain exponential tail bounds for the minimal eigenvalue
(sometimes called ground state) of the operator $\mathscr H$ as well as its
order of magnitude $\log L$ when the operator is considered on a large box
$\mathbb T_L:= \mathbb R^2/(L\mathbb Z)^2$ with $L\to \infty$.

Probability