  As it collapses to form a halo, the shape of a protohalo patch is deformed by
the initial shear field. This deformation is often modeled using the
"deformation" tensor, constructed from second derivatives of the gravitational
potential, whose trace gives the initial overdensity. However, especially for
lower mass protohalos, this matrix is not always positive definite: one of its
eigenvalues has a different sign from the others. We show that the evolution of
a patch is better described by the "energy shear" tensor, which is positive
definite and plays a direct role in the evolution. This positive-definiteness
simplifies models of halo abundances, assembly and of the cosmic web.
