  Here we investigate the mechanism of charge-disproportionation (CD) in BaBiO3
(BBO) using density functional theory under different crystal symmetries and by
employing strain as an external perturbation. The competition between Bi 6sp-O
2p (s-p) and O 2p-O 2p (p-p) charge-fluctuations decides the electronic ground
state, charge-disproportionation and bond-disproportionation (BD) in BBO. An
extended Hubbard Hamiltonian involving onsite (U) and long-range (V) coulomb
repulsions is also employed to ascertain the microscopic conditions for the
formation of the lone-pair on bismuth site. A strong tensile strain increases
p-p fluctuation and enhances negative-charge transfer (CT) character, while
strong compressive strain favors s-p fluctuation leading to more positive-CT
character. Bulk BBO is at the verge of positive and negative-CT regimes.
