**4. Conclusions**

The construction of the general analytical theory, which described the effect of the complicated microfield dynamics on the spectral line shape formation, faced the greatest difficulties. The Frequency Fluctuation Model is believed to be the most accurate and simplest method for account such effects. Therefore, it was necessary to examine this theory for different types of interactions. In the present paper, we tested the FFM for the van der Waals forces.

The FFM showed good results for moderate values of temperature and density. In the case of broadening by neutral atoms, it was no exception. The results of the FFM procedure were in agreemen<sup>t</sup> with the CT theory (Figure 1).

The FFM did not reproduce the impact width correctly (Figure 2). As it was shown in [17], for the linear Stark effect, this problem could be solved by accounting for the dependence of the jumping frequency on the field strength. Apparently, the resolution of the problem of the broadening by neutral particles consisted of the description of the complicated dynamics of the van der Waals field.

**Author Contributions:** Conceptualization, V.L., V.A. and A.L.; methodology, V.L.; software, A.L.; validation, V.L., A.L. and V.A.; formal analysis, V.L., V.A. and A.L.; investigation, A.L., V.A. and V.L.; resources, A.L.; data curation, V.L.; writing—original draft preparation, A.L.; writing—review and editing, V.L.; visualization, A.L.; supervision, V.L.; project administration, V.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
