His choral work Requiem (1963-1965) was another success, as were Ramifications (1968-1969), for string orchestra or 12 solo strings, and Clocks and Clouds (1972-1973). In 1972, Ligeti became Composer in Residence at Stanford University and the following year took on a professorship at the Hamburg Academy of Music. Ligeti composed his opera Le Grand Macabre in the period 1975-1977, but revised it in the 1990s, with the final version completed in 1997. It has become one of his most popular large works.