In eq. ( ), n is the number of particles, T is the temperature, p is the pressure and 7𝑅 *+ 9 are the internal coordinates that define the geometry. The subscript in eq. ( ) indicates that we consider 𝐺 as being differentiated at constant temperature, pressure and system geometry. The symbol 〈… 〉 represent ensemble averaging. The theory to be described will consider a grand canonical (𝜇𝑉𝑇) ensemble. Equilibrium in such an ensemble means that each system has reached the same chemical potential, temperature and volume. In other words, the free energy is at a minimum. Figure illustrates an example of an n-electron system surrounded by an environment that acts as an infinite electron reservoir with which the system can exchange electrons. A large collection of such open systems, all in equilibrium with a reservoir, is, by definition, a grand canonical ensemble. What is the connection between eq. ( ) and electronegativity? With the help of eq. ( ), the Gibbs free energy can be expressed within EQC as,