Yang, W; Lee, C; Ghosh, SK, *Molecular softness as the average of atomic softnesses: Companion principle to the geometric mean principle for electronegativity equalization*,
The Journal of Physical Chemistry, vol. 89 no. 25
(1985),
pp. 5412-5414 .
**Abstract:**

*It is demonstrated that, for a molecule consisting of M atoms, the relation, 1/(I - A) = (1/M)ΣiM[1/(Ii - Ai)], is valid to a reasonable approximation, where I and A are the ionization potential and electron affinity of the molecule and Ii, Ai denote the same quantities for the ith atom. This relation is the arithmetic average principle for molecular softness where the softness, S, is defined [W. Yang and R. G. Parr, Proc. Natl. Acad. Sci. U.S.A., 82, 6723 (1985)] as the inverse of the hardness (η) i.e. S = 1/η = 1/(I - A). The calculated values of I - A for 33 molecules show agreement with experimental results. The conditions for the validity of this principle are shown to be analogous to those for the geometric mean principle for electronegativity equalization in molecules. © 1985 American Chemical Society.*