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The inference of the underlying state of the plasma in the solar chromosphereremains extremely challenging because of the nonlocal character of the observedradiation and plasma conditions in this layer. Inversion methods allow us toderive a model atmosphere that can reproduce the observed spectra byundertaking several physical assumptions.  The most advanced approaches involve a depth-stratified model atmospheredescribed by temperature, line-of-sight velocity, turbulent velocity, the threecomponents of the magnetic field vector, and gas and electron pressure. Theparameters of the radiative transfer equation are computed from a solid groundof physical principles. To apply these techniques to spectral lines that samplethe chromosphere, NLTE effects must be included in the calculations.  We developed a new inversion code STiC to study spectral lines that samplethe upper chromosphere. The code is based the RH synthetis code, which wemodified to make the inversions faster and more stable. For the first time,STiC facilitates the processing of lines from multiple atoms in non-LTE, alsoincluding partial redistribution effects. Furthermore, we include aregularization strategy that allows for model atmospheres with a complexstratification, without introducing artifacts in the reconstructed physicalparameters, which are usually manifested in the form of oscillatory behavior.This approach takes steps toward a node-less inversion, in which the value ofthe physical parameters at each grid point can be considered a free parameter.  In this paper we discuss the implementation of the aforementioned techniques,the description of the model atmosphere, and the optimizations that we appliedto the code. We carry out some numerical experiments to show the performance ofthe code and the regularization techniques that we implemented. We made STiCpublicly available to the community.

STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations