The NCA diagrams correspond to the following analytic expression for the pseudoparticle self-energy
(1) |
To simplify the equations, we will write them in terms of spectral
quantities instead of Green's functions. Let us define
(3) | |||
(4) |
With the above definitions, the NCA equations simplify to
The local electron Green's function within NCA approximation is given
by
(6) |
Let me pause here for a moment and show that the
equations are invariant with respect to
the shift of frequency in the pseudo particle quantities due to local
gauge symmetry, therefore that appears in the definition of
the pseudo Green's functions can be an arbitrary number. In numerical
evaluation, we use this to our advantage and choose zero frequency at
the point where pseudo particle spectral functions diverge.
First, we want to show that shift of frequency arguments of all
pseudo-particle quantities by
does not affect the
equations provided that also changes to
(9) | |||
(10) | |||
(11) |
In numerical evaluation, we can either fix and calculate , or alternatively, fix and calculate at each iteration. The goal is to resolve the structure of the pseudo-particle functions and therefore it is best to choose the zero frequency at the point where the pseudo-quantities have a threshold (they diverge at zero temperature). In the program, user can fix to an arbitrary number and the corresponding , which is just the impurity free energy, is calculated. This guarantees that in zero temperature limit the threshold point is at zero frequency. For small number of local states, like for the one band model, the choice is best. When the number of bands grows, should grow as well. A good choice is where is the number of all local states.
The above NCA equations are still numerically intractable because of
exponential factors appearing in Eq. (7) and (8). As we
mentioned, the pseudo-quantities have a threshold, i.e., they
exponentially vanish below a certain frequency (which is zero
frequency if is a fixed constant). However, the exponential tails
obviously enter the equation Eq. (7) and need to be
calculated. The problem can be circumvented by defining new quantities
that coincide with the original quantities above the threshold, but are
nonzero also below the threshold and carry the information needed below
the threshold. We will use the tilde sign for those new quantities
(12) | |||
(13) |