Optical theorem for advanced DDA formulations

[GoogleCodeExporter]

Simple relation Cext=Cabs+Csca is automatically satisfied for default DDA 
formulation. However, it may get much more complicated for more advanced 
formulations, especially those that modify the interaction term between the 
dipoles. So this warrants a special study to test the current implementation of 
cross sections calculation in ADDA. Moreover, theoretical analysis is also 
required.

For instance, currently when using '-int igt_so' optical theorem is satisfied 
only up to O[(kd)^2].

Original issue reported on code.google.com by yurkin on 24 May 2012 at 9:00

[myurkin]

There should be always some analytical relation between full and far-field Green's functions, but it requires careful consideration.

/cc @Sunmosk