Theoretical Chemistry Accounts manuscript No. (will be inserted by the editor) Competition for water between protein (from Haloferax mediterranei) and cations Na + and K + : A quantum approach to problem. Ferrer, Juan · San-Fabi´ an, Emilio Received: May 9, 2017/ Accepted: date Abstract The competition between Na + and K + with a protein for water was investigated by using Density Functional Theory (DFT) calculations. The optimized potential energy curves have been made in the DFT, together with balanced basis sets of split valence Def2- SV(P). Initially, calculations were performed in order to know the organization of the hy- dration shell of the sodium and potassium ions, when up to sixteen molecules of water are added. The results indicate the structure and stability of these cations with water clusters. Then, this knowledge was used for the analysis of the hydrated protein when potassium or sodium cations approach to them, showing that cation has a dehydration process more favourable energetically, and indicating for which cation, potassium or sodium, is the com- petition with the protein for water more favorable. Ferrer, Juan Department of Agroqu´ ımica y Bioqu´ ımica, Universidad de Alicante. San Vicente del Raspeig, 03690 Ali- cante, Spain. E-mail: jferrer@ua.es San-Fabi´ an, Emilio Department of Qu´ ımica F´ ısica, Unidad Asociada CSIC-UA and Instituto Universitario de Materiales, Uni- versidad de Alicante, San Vicente del Raspeig, 03690 Alicante, Spain. E-mail: sanfa@ua.es