(Experimental) This variable selects the method used for the treatment of the
singularity of the Coulomb potential in Hatree-Fock and hybrid-functional DFT calculations.
This shoulbe be only applied for periodic systems and is only
used for FFT kernels of the Poisson solvers.
The singularity is replaced by zero.
The general treatment of the singularity, as described in Carrier et al, PRB 75 205126 (2007).
This is the default option
The treatment of the singulariy as described in Gygi and Baldereschi, PRB 34, 4405 (1986).
This is formally valid for cubic systems only.
The divergence in q=0 is treated analytically assuming a spherical Brillouin zone