  We consider the minimizers for the biharmonic nonlinear Schr\"odinger
functional $$ \mathcal{E}_a(u)=\int_{\mathbb{R}^d} |\Delta u(x)|^2 d x +
\int_{\mathbb{R}^d} V(x) |u(x)|^2 d x - a \int_{\mathbb{R}^d} |u(x)|^{q} d x $$
with the mass constraint $\int |u|^2=1$. We focus on the special power
$q=2(1+4/d)$, which makes the nonlinear term $\int |u|^q$ scales similarly to
the biharmonic term $\int |\Delta u|^2$. Our main results are the existence and
blow-up behavior of the minimizers when $a$ tends to a critical value $a^*$,
which is the optimal constant in a Gagliardo--Nirenberg interpolation
inequality.
