  A new numerical approach to entanglement entropies of the Renyi type is
proposed for one-dimensional quantum field theories. The method extends the
truncated conformal spectrum approach and we will demonstrate that it is
especially suited to study the crossover from massless to massive behavior when
the subsystem size is comparable to the correlation length. We apply it to
different deformations of massless free fermions, corresponding to the scaling
limit of the Ising model in transverse and longitudinal fields. For massive
free fermions the exactly known crossover function is reproduced already in
very small system sizes. The new method treats ground states and excited states
on the same footing, and the applicability for excited states is illustrated by
reproducing Renyi entropies of low-lying states in the transverse field Ising
model.
