Seraphic Fire's Mozart 'Requiem,' in New York

Of all the musical masterpieces left unfinished by their composers, none has had more attempts to complete it than Mozart's

Requiem

. The original completion by Mozart student Franz Xaver Süssmayr, commissioned by the composer's widow, has become standard in the concert hall.

Yet, as conductor Patrick Dupré Quigley says, "We will only get Mozart's music from Mozart."

So when he decided to commission a "completion" for his excellent Miami vocal ensemble, Seraphic Fire, he gave his composer three instructions: Be reverential, be referential, and don't try to mimic Mozart.

Quigley's choice was Gregory Spears, whose own choral music has a spare, mystical quality. After a set of concerts in Florida last weekend, Quigley, Seraphic Fire, and the Sebastians, a young period-instrument group from New York, took Mozart and Spears on a swing through the Northeast Corridor and was to perform Thursday night at St. Clement's Church in Center City.

Judging from Wednesday's performance at Trinity Church Wall Street, Spears has managed a remarkable feat. Though his setting of the "Sanctus," "Benedictus," and "Agnus Dei" (the only movements for which no Mozart material has survived) doesn't sound like Mozart at all, he's used the choir and the orchestra very much as Mozart did.

Quigley led a well-judged performance in New York: In the Mozart, his tempos were fast enough to keep momentum without blurring the notes in Trinity's resonant space, and the music's drama was held just enough in check to let Spears' more tormented passages, when they arrived, pack a wallop. The singing of Seraphic Fire was crystalline and committed (with excellent solo work by soprano Margot Rood and tenor Steven Soph), and, in this generous acoustic, the Sebastians' period instruments sounded wonderful.

For the program's first half, Quigley made an unusually well-thought-out selection of music, each work casting a different light on the Mozart-Spears collaboration to come.

Heinrich Schütz's 1648 funeral motet, "Selig sind die Toten," ("Blessed are the dead") was paired with Felix Mendelssohn's 1843 Psalm setting, "Richte mich, Gott," ("Do me justice, O God"). As with Mozart and Spears, the melodies and harmonies were each of their time, but the technique and approach were the same.

Knut Nystedt's "Immortal Bach" (1987) and Ingram Marshall's "Hymnodic Delays" (2001) used a different approach to the old, using late 20th-century techniques to take simple songs apart and put them back together. The singers' tuning was especially good here, making those tone clusters electric and tangy rather than ugly and abrasive.