Very few of the real giants of music cultivated the quality of geniality in their compositions. Baring their esthetic psyches, after all, was much too serious a matter. But a captivating exception was normally dour old Father Brahms.

Yesterday Wolf Trap opened its week-long Brahms festival, honoring the 150th anniversary of his birth, with some of this wonderful music, in this case for vocal quartet and piano. These little songs are works of spontaneous Viennese charm and were composed for performance by amateurs in the home. They sound light years away from, say, the majestic dimensions of the Brahms symphonies.

First there were the first "Liebeslieder" Waltzes, Op. 52, all 18 of them. The whole set lasts only about 30 minutes. And each little waltz goes down with the ease of a Viennese bonbon, particularly when performed with the grace shown yesterday. Four members of the Wolf Trap Chamber Singers--soprano Elizabeth Fulford, alto Gretchen Greenfield, tenor James McDonald and bass Richard Dirksen--sang with splendid rapport, and the four-hand piano part was buoyantly played by Colette Valentine and Edward Mattos.

Then came three songs for vocal quartet and single pianist, Op. 64, which added a slightly greater note of seriousness. But soon the singers were back to pure, and irrestible, Viennese (or rather gypsy) geniality with the "Zigeunerlieder." A delightful afternoon in the Barns.