It seems reasonably clear to me that, from a computational perspective, functional central planning is not practically possible.

For a great deal on this topic, inspired (as maybe nigerweiss's remark was too?) by Francis Spufford's novel Red Plenty, see this article by Cosma Shalizi.

He doesn't happen to say anything about externalities as such. But anyone who finds this interesting might enjoy reading Shalizi's piece.