The chemical hardness introduced by Pearson in 1960s is an extremely useful theoretical descriptor. It is apparent that principles such as hard and soft acids and bases (HSAB) and maximum hardness principle (PMH) related with the chemical hardness provide theoretical justifications and explanations for many issues in chemistry. In the present report, a new molecular hardness equation to calculate chemical hardness of functional groups and molecules is derived utilizing the relationship with charge of the electronic energy for atoms, Density Functional Theory, and Datta's global hardness equalization principle. The reliability of the new equation is assessed by comparison with experimental data, the results of Datta's geometric mean equation, and molecular hardness values obtained in the present work. The new equation provides the opportunity to calculate the chemical hardness of molecules from first ionization energies (IE1), first electron affinities (EA(1)) and second ionization energies (IE2) of atoms in the molecules. (C) 2015 Elsevier B.V. All rights reserved.