inter atomic bond distance in cubic system = a * SQRT(deltax^2 + deltay^2 + deltaz^2) where x y and z are the co ordinates of the atoms in the unit cell
ABO3 perovskite will have
8 A cations on each corner (8 x 1/8 = 1)
1 B cation in the centre (0.5, 0.5, 0.5)
6 O anions, one on each face (6 x1/2 =3)
AO distance is therefore 3.905A * SQRT(0.5^2 + 0.5^2 + 0^2) = 2.761A (assuming you use A = (0,0,0) and O =(0.5,0.5,0)
BO distance is 3.905A * SQRT(0^2 + 0^2 + 0.5^2) = 1.9525A (assuming you use B = (0.5,0.5,0.5) and O = (0.5,0.5,0)
