Deformations (i.e. the deformation matrix )
can either refer to
the previous time point ( -updated Lagrange formulation) or to
time 0 ( -total Lagrange formulation).
In both cases, TOCHNOG composes the deformation
tensor with a polar decomposition into
with
the deformation matrix,
the rotation matrix and
the stretch matrix.
In a -updated_without_rotation approximation,
the rotation matrix is neglected (this saves CPU time).
See also group_materi_memory.