Stanley Myers' classical guitar piece "Cavatina" which served as the theme for the film "The Deerhunter" was originally written for piano but was rewritten for guitar by Myers at the behest of John Williams. The word itself, "Cavatina" is the diminutive of «Cavata» and comes originally came from 18th-century when it denoted both a songlike piece of instrumental music and in opera a short aria. (You'll find it for, example three times, in Le nozze di Figaro (Marriage of Figaro (K. 492) and also in the scoring for the final movement of Beethoven’s String Quartet in B flat op.130).
Myers' composition for guitar (and indeed the piano piece) is characterised by a clear melodic line and expresses a gentleness and wistful regretfulness it works well for both instruments it also works wonderfully well as a choral piece. Judge for yourselves, below the fold you'll find a play list that consists of a performance by Stockholms Gosskör (Stockholms Boys Choir) conducted by their artistic director Karin Skogberg Ankarmo given on June 23rd 2010 during a joint concert with the St. Petersburg State Choir together with the guitar OFS and a very nice piano version uploaded and played by YouTube User TAUCE67. (Do play the piano version as well as the choral performance it's really beautifully played, if you like piano music TAUCE67's is well worth your while visiting it's a mix of well known melodies and his own compositions all of them very listenable to).