How? Is there any evidence at all that 'negotiation' can work in this case?
The fallacy in all the traditional thinkers is that they think possible to negotiate with people who think and believe they are acting with Divine authority. One cannot negotiate with God!
The only easy way to read current Kremlin thinking is to read Alexandr Dugin, several of whose books are here:
Reading these, mandatory reading at Russian military schools, will explain how 'negotiation' works in present Russian governmental environment.
Summary: don't even Think that they will stop before they have
all Russia:
That means everything that ever was part of the Russian Empire.
Negotiate away, I'm sure they'll settle for Sonoma County California, Alaska, bits of the rest of NorthWest North America, plus obvious Russia like Latvia, Lithuania, Estonia, Finland, Georgia, Hungary, and so on. Ukraine is strictly an internal conflict, after all, even though some of you unenlightened people don't know it.
Read Dugin, then you'll understand their thought processes.
End: There is no choice! Fight until they're defeated. Hope that they are sane enough to not totally destroy their own interests. Nobody knows if sanity can prevail, but there is no prospect for negotiation any more than there was with Hitler. This time there are quite a few 'true believers' who will continue after Putin would be deposed. I really, really hope I'm wrong...but, living among 'true believers' for long parts of my life convinces me otherwise.