I use Gaussian 09 to obtain the Hamiltonian and overlap matrices, $H_0$ and $S_0,$ where the B3LYP functional and the 6-31G basis set are adopted. Then how to transform the system to an orthogonal basis using Löwdin transformation?
$$ \begin{align} H_1 &= S_0^{-\frac{1}{2}}H_0S_0^{-\frac{1}{2}} \\ H &= U^\dagger H_1U \end{align} $$