To account for the stability of a molecule, Pearson  has introduced an important parameter, viz., chemical hardness. For an N-electron system, the second derivative of energy with respect to N, keeping external potential v r( ) xed, is considered to be a measure of the chemical hardness :
η = ∂ ∂
The hardness can be expressed in terms of ЄHOMO and ЄLUMO as follows :
Electronegativity (χ)  (negative chemical potential [μ] ) for an N-electron system is de ned as the rst-order number derivatives of total energy (E) as follows:
χ −µ= = ∂ ∂
where v r( ) is the external potential. Using a nite difference approximation, χ may be expressed as 
χ = +I A
The nucleus-independent chemical shift (NICS) has been de ned as  “absolute magnetic shieldings computed at ring centers (nonweighted mean of the heavy atom coordinates).” The NICS is considered as the negative value of the magnetic shielding at a chosen point in a molecule. In general, a small value (negative or positive) of NICS identi es a system as nonaromatic, whereas a signi cant negative NICS value indicates the system as aromatic and a signi cant positive NICS value classi es a system as antiaromatic.