Modern completions of Mozart's Requiem
This article lists some of the modern completions of the Requiem by Wolfgang Amadeus Mozart.

Liturgical completions 
For the first performance of the Requiem in Rio de Janeiro on December 1819, Austrian composer Sigismund von Neukomm constructed a movement based on material in the Süssmayr version. Incorporating music from various movements (including the "Requiem aeternam", "Dies irae", "Lacrymosa", and "Agnus Dei"), the bulk of the piece is set to the "Libera me", a responsory text traditionally sung after the Requiem Mass, and concludes with a reprise of the "Kyrie" and a final "Requiescat in pace". A contemporary of Neukomm and a pupil of Mozart's, Ignaz von Seyfried, composed his own Mozart-inspired "Libera me" for a performance at Ludwig van Beethoven's funeral in 1827.

The "Amen" fugue 
In the 1960s, a sketch for an "Amen" fugue was discovered, which some musicologists (Levin, Maunder) believed Mozart intended as a conclusion of the sequence after the "Lacrymosa". H. C. Robbins Landon argued that the "Amen" fugue was not intended for the Requiem, but rather "may have been for a separate unfinished mass in D minor" to which the Kyrie K. 341 also belonged.

There is, however, compelling evidence placing the "Amen" fugue in the Requiem based on current Mozart scholarship. First, the principal subject is the main theme of the Requiem (stated at the beginning and throughout the work) in strict inversion. Second, the fugue is found on the same page as a sketch for the "Rex tremendae" (together with a sketch for the overture of his last opera, The Magic Flute), and thus dates from late 1791. The only instance of the word "Amen" occurring in anything Mozart wrote in late 1791 is in the Requiem sequence. Third, as Levin points out in the foreword to his completion, the addition of the "Amen" fugue at the end of the sequence would maintain an overall pattern that closes each large section with a fugue, a design that appears intentional.

Completions since the late 20th century 
Since the 1970s several composers and musicologists, usually dissatisfied with the traditional "Süssmayr" completion, have attempted alternative completions of the Requiem. Each version follows a distinct methodology.

Karl Marguerre published an essay on Süssmayr's passages in 1962, replacing a few bars in the middle of the "Lacrymosa", "Sanctus", and "Benedictus" with quotations from other Requiem movements. Marguerre also extended the instrumentation given by Süssmayr to include high woodwinds (oboe, clarinet, flute).
Marius Flothuis tried to repair the most obvious shortcomings in Süssmayr's completion, such as the overly long trombone solo in "Tuba mirum", potentially excessive use of trumpets, timpani, and trombones, and the strange key choice of the reprise of the "Osanna" fugue. Flothius's completion was not published but recorded by Jos van Veldhoven.
Franz Beyer revised Süssmayr's orchestration toward a more Mozartian style and introduced minor changes to Süssmayr's sections (e.g., slightly lengthening the "Osanna" fugue for a more conclusive-sounding ending).
Hans-Josef Irmen replaced the "Amen", "Sanctus", and "Agnus Dei" with parodies of Mozart's earlier works.
H. C. Robbins Landon orchestrated parts of his completion using Eybler's partial work, thinking Eybler's work represents a more reliable guide of Mozart's intentions.
Richard Maunder completely rewrote the orchestration working from Mozart's autographs and eliminated Süssmayr's portions except for the "Agnus Dei". He recomposed the "Lacrymosa" from bar 9 onwards and incorporated a completion of the "Amen" fugue.
Duncan Druce made extensive changes to the orchestration, retained Eybler's ninth and tenth measures of the "Lacrymosa", and substantially lengthened the movement to end in an extended "Amen" fugue. He also rewrote the "Benedictus" using the opening theme as its starting point.
Robert Levin retained the structure of Süssmayr's orchestration and contributions while adjusted orchestration, voice leading, and other instrumental passages to achieve a more Mozartean sound. Other notable features included a completion of the "Amen" fugue and an extension of Süssmayr's "Osanna" fugue.
Knud Vad followed Süssmayr's completion until the "Sanctus" and "Benedictus", which he rewrote in places (e.g., "Osanna" turned into a double fugue played adagio).
Simon Andrews followed a similar method to Levin's but was considerably less radical in adding new material.
Pánczél Tamás revised Süssmayr's score similar to Beyer but significantly extended the "Lacrymosa" past Süssmayr's passages to end in a completion of the "Amen" fugue, and rewrote the "Benedictus" ending to lead into the "Osanna" reprise.
Clemens Kemme rewrote the orchestration in a style closer to Eybler's, emphasizing the basset horns in particular, and reworking the "Sanctus", "Benedictus", and extended "Osanna" fugue.
Michael Finnissy used Süssmayr's orchestration as its basis but eliminated Süssmayr's compositions. Of Finnissy's five revised movements, the "Lacrymosa" was given a more conspicuously Mozartian style while the four final movements explored musical styles developed since Mozart's death in 1791.
Brett Abigaña revised Süssmayr's version and provided a new "Amen" fugue.
Benjamin-Gunnar Cohrs provided an entirely new instrumentation based on Eybler's ideas, new elaborations of the "Amen" and "Osanna" fugues, and a new continuity of the "Lacrymosa" (after bar 18), "Sanctus", "Benedictus", and "Agnus Dei", employing those bars which Cohrs speculated Mozart might have sketched himself.
Timothy Jones followed a Levin-like approach in reworking the "Lacrymosa" and composing an extensive "Amen" fugue modeled on the "Cum Sancto Spiritu" fugue from the Great Mass in C minor, K. 427. He applied the same process to the "Sanctus" and "Osanna" fugue.
Gregory Spears like Finnissy, included a new "Sanctus", "Benedictus", and "Agnus Dei" to replace the Süssmayr completion of those movements. Spears's completion recognized the juxtaposition of old and new sources common in liturgical music of the period and incorporated two cadential fragments from Süssmayr's completion into the end of the "Benedictus" and "Agnus Dei".
Michael Ostrzyga corrected Sussmayr's work like Levin, but retained some of it, completing the "Amen" fugue and composing further Mozartean material in discrete moments.
Masaaki Suzuki and Masato Suzuki followed a methodology similar to Robbins Landon, but elaborated further on Süssmayr's sections. They also included a short "Amen" fugue.
Gordon Kerry was commissioned by the Australian Broadcasting Corporation to write a completion. He brought new additions to the manuscript, but kept the overall proportions of Süssmayr's version. 
Pierre-Henri Dutron revised Süssmayr's version. He rewrote the "Sanctus" and "Benedictus" from opening themes onward and took creative liberties concerning the vocals between the chorus and soloists. Conductor René Jacobs used Dutron's completion for performances in 2016.
R. C. Keitamo provided a new orchestration influenced by motivic material from other late Mozart works, reworked the "Lacrymosa", and completed the "Amen" fugue. Like Maunder, this edition dispensed with Süssmayr's "Sanctus", "Osanna", and "Benedictus". The "Agnus Dei" was a parody of the Kyrie in D Minor, K. 341, adapted to the Requiem's instrumentation.

Many composers attempting a Requiem completion used the sketch for the "Amen" fugue discovered in the 1960s to compose a longer and more substantial setting for concluding the sequence. In the Süssmayr version, "Amen" is set as a plagal cadence with a Picardy third (iv–I in D minor) at the end of the "Lacrymosa". Only Jones combined the two, ending the fugue with a variation on the concluding bars of Süssmayr's "Lacrymosa" as well as the plagal cadence.

Notes