Yes sir, I agree. I do not have any proof to show that the resonance peak at 630nm should not exist. Instead I was trying to understand the cause of this peak. I am using periodic boundary condition and the period is 630nm in this case (and I see the spike exactly at this wavelength). When I change the period, the position of spike also shifts to the wavelength corresponding to period (but I do not see such a huge shift in the LSPR peak at 670nm). The dimensions of nanostructure do not seem to affect the spike as it does to the peak at 670 nm. Would this spike be due to surface lattice resonance?. Also when I reduce the period to a wavelength far away from the LSPR peak (670nm) the spike start to fade and it becomes almost nil when the period is 450nm. When PML boundary condition is used I do not see this spike. These are some features that I observed regarding this spike and I am curious to know why do I see this spike. I will try as you suggested.