ABINITIO CALCULATION OF THE CHARGE AND LATTICE MODULATION IN BABIO3

Total energy calculations for BaBiO3 have been carried out using local density functional theory and the full-potential LMTO method. Considering also the 5s2-states of Ba as valence states in a second energy window the total energy is calculated as function of the two unstable frozen phonons at the R-point, namely, the breathing GAMMA'2 and the [1 1 0]-component of the rotational GAMMA-25 mode. The absolute minimum corresponds to a rotation angle of 9.6 degrees and oxygen displacements due to the breathing mode of u = 0.025a where a is the theoretical cubic lattice constant equal to 8.151 a.u. Our results show that the observed modulation in BaBiO3 can be quantitatively understood within the LDA. Charge density plots of modulated ground states are also given and discussed.