  Using the density matrix renormalization group technique, we evaluate the
low-energy spectrum (ground state and first excited states) of the anisotropic
antiferromagnetic spin-one-half chain under magnetic fields. We study both
homogeneous longitudinal and transversal fields as well as the influence of a
transversal staggered field on opening of a spin-gap. We find that only a
staggered transversal field opens a substantial gap.
