(7.23) |

The cohesive energy thus becomes

= | |||

= | (7.24) |

Note that the first term contains only off-diagonal elements of the Hamiltonian matrix while the second term contains on-site elements. The forces acting on the atoms are then obtained by differentiating the cohesive energy with respect to atomic positions, that is

= | (7.25) | ||

= | (7.26) |

When self-consistency is required (see above), or when the on-site elements of the Hamiltonian are constant, the second term appearing in the forces vanishes, and we obtain

(7.27) |