${\bf A}$-Matrix Setup:

The ${\bf A}$ matrix elements are calculated as the sum of the contributions of the associated basis grid points of the segments $k$ and $l$ if their distance is below a certain threshold, the centers of the segments are used otherwise. For all segments that do not have associated basis grid points, i.e. intersection seam segments, the segment centers are used. The diagonal elements $A_{kk}$ that represent the self-energy of the segment are calculated via the basis grid points contributions, or by using the segment area $A_{kk} \approx 3.8 \sqrt{S_i}$, if no associated basis grid points exist.