  We present an equation of state and radiative opacities for a strongly
magnetized hydrogen plasma at magnetic fields B, temperatures T, and densities
\rho typical for atmospheres of isolated neutron stars. The first- and
second-order thermodynamic functions, monochromatic radiative opacities, and
Rosseland mean opacities are calculated and tabulated, taking account of
partial ionization, for 8x10^{11} G < B < 3x10^{13} G,
  2x10^5 K < T < 10^7 K, and a wide range of \rho. We show that bound-bound and
bound-free transitions give an important contribution to the opacities at T <
(1-5)x10^6 K in the considered range of B in the outer neutron-star atmosphere
layers, which may substantially modify the X-ray spectrum of a typical
magnetized neutron star. In addition, we re-evaluate opacities due to free-free
transitions, taking into account the motion of both interacting particles,
electron and proton, in a strong magnetic field. Compared to the previous
neutron-star atmosphere models, the free-free absorption is strongly suppressed
at photon frequencies below the proton cyclotron frequency.
  The latter result holds for any field strength, which prompts a revision of
existing theoretical models of X-ray spectra of magnetar atmospheres.
  Note added in v4: for free-free photoabsorption (Appendix B), equivalent but
much simpler expressions are available in Appendix B of Potekhin & Lai,
arXiv:astro-ph/0701285.
