In principle, we could extend molecular orbital theory to larger molecules by the same procedures: combine the atomic orbitals from all atoms in the molecule into an equal number of MO's that extend over the entire molecule, then fill these MO's from the lowest energy up with as many electrons as are available. In practice, this rapidly exceeds the calculating abilities of even the best digital computers, and some compromises and approximations are called for. The most important of these is the localized molecular orbital approximation, which brings us back from entire molecules to bonds between pairs of atoms. The localized MO theory assumes that we can combine an AO from each of two atoms that share a bond to produce bonding and antibonding localized MO's extending only over the two atoms. If the bonding MO is filled with a pair of electrons, then a bond is formed between the atoms. The other atoms in the molecule, and even the other atomic orbitals on the two being bonded, are assumed not to matter very much to that particular bond.