In terms of chords and melody, ACID has complexity on par with the typical song, having near-average scores in Chord-Bass Melody and below-average scores in Chord Complexity, Melodic Complexity, Chord-Melody Tension and Chord Progression Novelty.
ACID is written in the key of C♯ Phrygian. According to the Theorytab database, it is the 9th most popular key among Phrygian keys and the 60th most popular among all keys. The C♯ Phrygian scale is similar to the C♯ Minor scale except that its 2nd note (D) is a half step lower. Chord progressions in Phrygian often rely on the major chord built off of this 2nd scale degree (D major) which gives the key its distinctive sound. See the C♯ Phrygian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!