**4. Conclusions**

The main goal of this article was to present a simple ab initio-based approach to evaluate Δ *λmax* caused by substitution of charged or polar amino acids in visual and microbial rhodopsins. If the rhodopsin three-dimensional structure is available, Δ *λmax* can be obtained from the plots and tables given in the article and Supplementary Materials. The performance of the proposed models is evaluated against a test set consisting of ten mutants of bovine and sodium pumping rhodopsins.

Additional, general conclusions of this study can be summarized as follows:



The models proposed in this study can be used to estimate the direct part of Δ *λmax* caused by residue substitution and, therefore, can be utilized both for the interpretation of experimental data and for the rational design of rhodopsins with specific spectral properties.

**Supplementary Materials:** Supplementary materials can be found at www.mdpi.com/xxx/s1.

**Author Contributions:** Conceptualization, M.N.R., A.V.V., Y.S.T.; investigation, data curation, A.A.S., D.M.N., V.N.M., M.S.P.; formal analysis, M.N.R., A.V.V., Y.S.T.; writing—original draft preparation, D.M.N.; writing—review and editing, M.N.R., A.V.V., Y.S.T.; supervision, project administration, M.N.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was funded by the Russian Science Foundation (RSF), gran<sup>t</sup> No. 20-13-00303.

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

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Acknowledgments:** Authors acknowledge HPC computing resources at the Resource Center "Computer Center of SPbU".

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
