Stokes NLTE Analytically Powered Inversion (SNAPI) code.

SNAPI is developed by Dr. Ivan Milic (originally MPS, now KIS) and Dr. Michiel van Noort (MPS), as a code for synthesis and inversion of NLTE spectral lines. The code was born out of a new idea for the calculation of the response functions for NLTE lines, that relies on the explicit inclusion of the Lambda operator in the spatial coupling between different atmospheric points. This way, response functions can be calculated almost as quickly as in LTE. The code is written in C++ and suitable for massively-parallel calculations. It has been used for the analysis of various spectral lines so far but mostly Sodium I D lines and Mg I b2 spectral lines, as well as various NLTE lines. 

The code is publicly available at: https://github.com/ivanzmilic/snapi

The calculation of response functions is described in Milic I., van Noort M. (2017), A&A, 601, 100, and the inversion code itself in Milic, I., van Noort M., (2018), A&A, 617, 11