for a quiver with potential, we can associate a vanishing cycle to each representation space. if there is a nice torus action on the potential, the vanishing cycles can be expressed in terms of truncated jacobian algebras. we study how these vanishing cycles change under the mutation of derksen-weyman-zelevinsky. we use ringel-hall algebras as the main organizing tools. the wall-crossing formula leads to a categorification of quantum cluster algebras under the assumption of existence of certain potential. this is a special case of a. efimov's result, but our approach is more concrete and down-to-earth. we also obtain a formula relating the representation grassmannians under sink-source reflections. in this talk, i will start with basic definitions of mutations of quivers with potentials, vanishing cycles, and hall algebras.