[13]SNUBBER CAPACITOR CALCULATION 

The idea of a snubber is that the capacitor absorbs the inductive energy stored in the load at the moment the switch (photomos) opens, and its value must be large enough so that the voltage across it does not exceed the rating of the switch.
The resistor is there partly for damping and partly to make sure that the capacitor doesn't discharge instantly through the switch the next time it closes. Its resistance value and its power rating must be deduced from the worst-case conditions for both scenarios. The power rating is also related to how frequently the switch is going to be operating.