Please refer to overcome the diverging issue:
Troubleshooting diverging simulations in FDTD
No you cannot change the simulation time bu checkpoint. It can only change the autoshutoff value. This is why I suggest you to use much longer simulation time.
No you cannot change the monitor settings without redoing the simulation.
I am not sure how much memory it needs. Usually the simulation time should be much less than what you said, for device ~300um. You may need to upgrade your computer to have better performance: Information on Hardware SpecificationsÂ
The computer and the software are the tools for you to do your projects. if the tools are not proper, your progress on the projects may be hindered.
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