In FORCE calculations, the transition dipole moment
printed (T-DIPOLE) represents the electric dipole moment associated with the
transition between the ground state and the vibrationally excited state.
The units are Debye. To understand how it is calculated, consider a
diatomic A-B, with R_{A-B} = 2.0
Ångstroms, and the partial charge on Q_{A} = -Q_{B} = 0.5
electrons. The ground-state dipole is 4.803*(R_{A-B} )*(Q_{A})
or 4.803 Debye. If the atoms A or B have p-orbitals, then
adjustments would need to be made for hybridization effects, these would be a
few percent of the dipole terms arising from the partial charges.

During the calculation of the Hessian matrix,
each atom is displaced in the X, Y, and Z directions.
From these displacements, the derivative of the dipole with respect to motion in
these directions, dD/dX_{A}, dD/dY_{A}, and
dD/dZ_{A}, can readily be calculated. Later on, the normal modes
are generated. The associated normalized eigenvectors,
Ψ, represent the motion
of the atoms in the normal modes. This allows the components of the transition
dipole, d_{1n}, to be calculated using: