Non-local formulation

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We have been researching the possibility to make non-local extension of the 
DDA. Now (r1304) there are two options implemented: 'nloc' and 'nloc_av' (as 
arguments to options -pol and -int, which are based on Gaussian densities in 
static limit.

Still, a lot need to be done to make something useful out of it.

Original issue reported on code.google.com by yurkin on 4 Feb 2014 at 1:42

[myurkin]

r1304 - a85f1ce3c2a271722c78140a7476af40b4b547c0

[myurkin]

/cc @fabiods

[myurkin]

Gaussian densities have been used in Lim J., Kang S., Kim J., Kim W.Y., and Ryu S. Non-empirical atomistic dipole-interaction-model for quantum plasmon simulation of nanoparticles, Scientific Reports 7, 15775 (2017). together with empirical adjustment of polarizabilities (based on coordination factor of a dipole).

[myurkin]

Another possible way to describe non-locality is through the Feibelman parameters, see, e.g., the following:

1. Hohenester U. and Unger G. Nanoscale electromagnetism with the boundary element method, Phys. Rev. B 105, 075428 (2022).
2. Yang Y., Zhu D., Yan W., Agarwal A., Zheng M., Joannopoulos J.D., Lalanne P., Christensen T., Berggren K.K., and Soljačić M. A general theoretical and experimental framework for nanoscale electromagnetism, Nature 576, 248–252 (2019).

However, it is not clear, whether it is feasible to implement this into the DDA.