  We present results of LDA calculations (band structure, densities of states,
Fermi surfaces) for possible iron based superconductor BaFe2Se3 (Ba123) in
normal (paramagnetic) phase. Results are briefly compared with similar data on
prototype BaFe2As2 and (K,Cs)Fe2Se2 superconductors. Without doping this system
is antiferromagnetic with T_N^{exp}~250K and rather complicated magnetic
structure. Neutron diffraction experiments indicated the possibility of two
possible spin structures (antiferromagnetically ordered "plaquettes" or
"zigzags"), indistinguishable by neutron scattering. Using LSDA calculated
exchange parameters we estimate Neel temperatures for both spin structures
within the molecular field approximation and show \tau_1 ("plaquettes") spin
configuration to be more favorable than \tau_2 ("zigzags").
