Dear all

I am checking my result of the DDK perturbation preparing for nonlinear optical calculation. It is a single-step calculation that only includes the DDK for the ground state.

According to the tutorial, the parameter ‘f-sum rule ratio’ should be close to 1. However, it is only 0.6 for my single-step calculation. I am wondering what is the physics meaning of this f-sum. Is it something related to the Thomas–Reiche–Kuhn sum rule?

Another question is how the DDK is calculated in ABINIT. In optical calculation, the matrix element of d(H)/d(k) is applied, so I would like to know what method is applied to calculate this matrix in ABINIT.

Best regards,