The screening interaction is defined as
The first term on the r.h.s. gives the exchange self-energy (XX), while the second term gives the frequency dependent part of the screening interaction. Starting from the non-interacting Green's function described in G a first approximation to the (irreducible) response function is constructed
This is used to construct the screened interaction W, and finally, the self-energy operator. At this point the Dyson equation should be solved self-consistently (to be conserving). As this scheme would be computationally too demanding, practical schemes neglects or approximates the self-consistency. Moreover approximations are introduced to make feasible the calculation of the screened interaction W.
with the frequency dependent screening function defined as
Using the plasmon pole approximation for the screening function (Xp) it is possible to obtain an analytical expression for the matrix elements of the mass operator:
The maximum vectors in the reciprocal lattice summation is determined by NGsBlk , while the range of the bands summation is given by GbndRnge . QPkrange (or alternatively QPErange ) selects the matrix elements to calculate.
can be solved by using either Newton or the secant root-finding algorithm ( DysSolver set to 'n' or 's'). The Newton method uses the first order of the Taylor series of a function in the vicinity of a root to evaluate the correction to the initial guess:
This leads to
with Z (the renormalization factor) defined
The derivative of the self-energy with respect to the frequency is calculated from finite differences. The flag NewtDchk enables the test on the convergence of this quantity. The secant method: nothing yet