Bond-bending force constant of silicon by an r → -space method

A method of calculating Wannier functions in - > r space to treat the bond-bending force constant, C11 -- C12, in Si is discussed. The Pauling hybrids are found to rotate in the opposite direction to the bonds when the crystal is distorted, and this results from the wave-function overlap with nearest neighbors. This counter-rotation has a marked symmetry-lowering effect on the charge distribution of a single bond, a smaller symmetry-lowering effect on the bond quadrupole moment and a very small effect on the crystal quadrupole moment/unit cell.8 refs.--AA