Pathfinder has a complicated system of damage and effects using positive energy and negative energy. Effects that deal negative energy damage harm the living and heal the undead while effects that deal positive energy damage heal the living and harm the undead, except when they do not heal. Under no circumstances does positive energy harm the living or negative energy harm the undead, except in 3pp that introduces just that.
By contrast, positive and negative energy effects simply do not exist in D&D 5e. Instead, necrotic and radiant are standard damage types while healing spells do not affect constructs or undead. I prefer this system for its simplicity, additional possibilities for monsters (e.g. starving wraiths falling to cannibalism dealing necrotic damage to each other), and because the positive/negative distinction is logically questionable.
This makes some conversions from Pathfinder to 5e a bit difficult. Most of the time it is easy enough to swap in necrotic, radiant or other damage types, but other times not so much. For example, a Pathfinder monster with "negative energy affinity" is not undead but reacts to positive/negative energy effects as if it was while a monster with the "light to dark" power can temporarily reverse how positive/negative energy affects it.
These effects would be pointless to convert. It seems a waste to just drop them completely, but I am hard pressed to devise an equivalent.