Minor change of resonate frequency is normal when you change the wavelength start due to FDTD's dicretization. More specific, the resonate frequency may change on the mesh size. In general, for most dispersive or non dispersive materials, it is the "wavelength start" that determines the actual meshing size. More general, the simulation accuracy depends on the mesh size. This is why we suggest users to use coarse mesh to get rough result quickly, and then using finder mesh to improve the accuracy later, provided other settings such as PML thickness also change accordingly to have the best performance. This is the convergence test. Please refer this link
https://support.lumerical.com/hc/en-us/articles/360034915833-Convergence-testing-process-for-FDTD-simulations
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