What is the name of the piano piece that Seven of Nine plays for the holographic Commander Chakotay in Season 7, Episode 18 (Human Error) of Star Trek: Voyager?
She plays it for him in a scene where Chakotay asks her to play without the metronome.
It is not the same piece she plays in the opening scene. In that scene she plays with a metronome and Chakotay is not present. (As already correctly pointed out, that piece is Chopin's Nocturne Op.72 No.1 in E minor.)