A theoretical study was conducted to determine the ionization constants of N-substituted amino acid derivatives using HF, DFT, and MP2 calculation methods. The extent of compatibility of these methods was determined by discussing the theoretical variables calculated in the three methods mentioned above, the relationship between the calculated physical variables have been found to be theoretical and to determine their nature. These variables were then correlated with the known chemical values of amino acids as pKa ionization parameters. The results obtained by this relationship were found to be good. This is indicated by the results of the statistical analysis across the correlation coefficient values. The theory that gave the best agreement between the values of the theoretical and the experimental ionization parameters were the MP2 method with good correlation coefficient (0.997) and standard error (0.162). As well as the large overlap between pKa values calculated theoretically with practical values where the difference (0.008) gives the opportunity to apply these variables in other studies. * Corresponding Authors

It was a theoretical study to calculated values of ionization constant of a group of amino acid derivatives using five methods of quantum mechanics for the purpose of comparison, two methods used semi empirical represented by a way of AM1, PM3 and the three methods used ab-initio basic calculation methods :HF, DFT and MP2, the compatibility of these methods with chemical bases known by discussing the theoretical variables were calculated by the five ways mentioned above, The results showed that the ab initio calculations were more accurate than semi empirical method which were created relationship between physical variables theoretically calculated with each other and determine their nature. Then link between these variables with known chemical values of amino acids ionization constants PK1. Results obtained through this relationship was good and that's what indicated to him the results of statistical analysis through correlation coefficient values, including the method of the DFT where it was