# Hessian of function

I need to calculate the Hessian of the following function: $f(x) = \ln\sum\limits_{a \in A}e^{\langle x,a\rangle}$

where $x \in \mathbb R^n$ and $A$ a set of $n$ dimensional vectors.

I calculated the gradient: $\nabla f(x) = \dfrac{\sum\limits_{a \in A} e^{\langle x,a \rangle} \cdot a }{\sum\limits_{a \in A} e^{\langle x,a \rangle}}$ but I don't know how to proceed!

Can anyone help me?