  Observed X-ray spectra of some isolated magnetized neutron stars display
absorption features, sometimes interpreted as ion cyclotron lines. Modeling the
observed spectra is necessary to check this hypothesis and to evaluate neutron
star parameters.We develop a computer code for modeling magnetized neutron star
atmospheres in a wide range of magnetic fields (10^{12} - 10^{15} G) and
effective temperatures (3 \times 10^5 - 10^7 K). Using this code, we study the
possibilities to explain the soft X-ray spectra of isolated neutron stars by
different atmosphere models. The atmosphere is assumed to consist either of
fully ionized electron-ion plasmas or of partially ionized hydrogen. Vacuum
resonance and partial mode conversion are taken into account. Any inclination
of the magnetic field relative to the stellar surface is allowed. We use modern
opacities of fully or partially ionized plasmas in strong magnetic fields and
solve the coupled radiative transfer equations for the normal electromagnetic
modes in the plasma. Spectra of outgoing radiation are calculated for various
atmosphere models: fully ionized semi-infinite atmosphere, thin atmosphere,
partially ionized hydrogen atmosphere, or novel "sandwich" atmosphere (thin
atmosphere with a hydrogen layer above a helium layer. Possibilities of
applications of these results are discussed. In particular, the outgoing
spectrum using the "sandwich" model is constructed. Thin partially ionized
hydrogen atmospheres with vacuum polarization are shown to be able to improve
the fit to the observed spectrum of the nearby isolated neutron star RBS 1223
(RX J1308.8+2127).
