where Δ Δ Φ Φ is the jump of electric displacement flux all over the integration surface, E n ( r ) is the component of microscopic electric field normal to the integration surface, ε ε r ( r ) is the local relative complex permittivity which takes the value ε ε m inside the picked metal particle, the value ε ε d inside the picked dielectric particle and the value ε ε e f f outside the picked particle, E 0 is the normal component of the macroscopic electric field. Formula (4) comes out of Maxwell's equality div ⁡ ⁡ ( ε ε r E ) = 0 . Thus only one picked particle is considered in Bruggeman's approach. The interaction with all the other particles is taken into account only in a mean field approximation described by ε ε e f f . Formula (3) gives a reasonable resonant curve for plasmon excitations in metal nanoparticles if their size is 10 nm or smaller. However, it is unable to describe the size dependence for the resonant frequency of plasmon excitations that are observed in experiments