Years before musician/composer Atticus Ross became an official member of Trent Reznor's longstanding Nine Inch Nails project, the pair had fostered a fruitful studio relationship. Mixing Ross' knack for atmospheric composition and Reznor's mastery of dark, machine-driven pop, their collaborative soundtrack for The Social Network packs all sorts of melancholy: On “Hand Covers Bruise,” a delicate piano plunges into foreboding low-end tones; an uptempo house beat ticks insistently as it fights with woozy pads and synth arpeggios on “In Motion.” For the climate-change documentary Before the Flood, Reznor brings a soul-baring lyrical elegance to the contemplative “A Minute to Breathe."