Gifting an AI empire (instant trade offer with nothing in return) can now (2.2.3+) increase its opinion of you and create an option modifier of up to +100 for 2 years ("favorably trade deal"). This works for all elements including resources and systems and is a very useful diplomatic tool. It allows you to "bribe" an AI empires can be "bribed" into agreeing to deals with you. At least temporary.
Here are some useful scenarios for this tactic:
- Quickly increase the opinion of an AI to get a deal (e.g. commercial contract)
- Gifting a new contact from the get go increases your chances to form a good relationship by starting out with a positive overall opinion
- Fixing a relationships, e.g. after you declined a war request from a federation ally (which gives you -100 for reach decline)
- Persuade an AI to open border for you (e.g. helpful if you are in a war and can't get to some of your enemies systems, because they are behind territory of another AI that has closed border to you)
Note that the "favorably trade deal" modifies will decay over time. So if the fundamentals are negative this will only work temporarily and you need to repeat the process after some months to keep its opinion positive. (or have established another relation that helps increase opinion, e.g. a research contract).
Also note that the total amount of opinion increase depends a lot on the value of the resource, the amount and the year of the game (e.g. 1000 minerals will give you almost 100 in the beginning, but may only yield 1 in the end game) and what kind of resources the AI currently needs. So its worth trying out different different resources. e.g. strategic resources will in my experience yield higher opinion then a normal resource for the same value.
Note that you can directly see how much the AI likes your offer in the trade acceptance indicator in the middle of the window.
Finally, if an AI does not like your offer at all you will get an trade acceptance of -1000. e.g. often if you try to buy a system that the AI owns or if you try to gift a colonized machine world to a fallen empire.