THERE are instances in which a piece of museum architecture seems as important as the art it is intended to house. This is the reaction visitors to the West Wing of the National Gallery in Washington may have on first seeing the vast space of the foyer with the Calder mobile floating in it.It was also the reaction I had on first seeing the new Museum of Modern Art (MMK) in Frankfurt, Germany. Filling a small, triangular block close to the city's center, this modern building designed by Viennese architect Hans Hollein, with its echoes of Frankfurt's familiar red sandstone, seems relatively uninviting on the outside. But step inside the building, which just opened this summer, and there is one pleasant surprise after another. It is difficult to see how so many different-appearing spaces, inc luding several major areas, could exist within the exterior walls. Frankfurt museums are generally open to the public free of charge, a fact which fits with the city's rich prewar cultural tradition and certainly does not hinder one's development of art appreciation. This latest museum, whose permanent collection commences in time with American Pop Art from the 1950s and 1960s, is to be devoted entirely to the work of contemporary artists. Rolf Lauter, chief curator of the MMK, spoke about the role of modern art. "Many of these works will not be seen to be important 50 years from now," he says. But that does not diminish their vitality for today's viewer. As Dr. Lauter explains it, modern art does not proceed in a line of succession similar to the transition from Impressionism to Post-Impressionism, for instance. The continuity with the past has been broken. Rather, today's artists are each expressing their reactions to the totality of modern life. "Their only common link is that their work flows out of their reaction to the world in which they live," he says. Thus, the 35 or 40 separate viewing areas in the MMK, some of them quite small, are appropriate for a group of artists whose works have little or no relation to each other. The two main criteria used by the museum's director, Jean-Christophe Ammann, who came here from the Kunsthalle in Basel in 1987, were that the MMK should provide "environments" for an artist's work. This implies more than a wall on which to hang a painting. And whenever possible, enough of an artist's work should be made available for the viewer to understand his or her message. The MMK achieves both. The American artist Bill Viola's work here, for instance, constitutes a total environment. One enters a dark room and finds himself surrounded by images projected on all four sides. In the center of the room a hushed voice whispers; suddenly, as a clash of sounds disturbs the peace, all four images change. The images, including postwar pictures from Frankfurt, were created for the MMK. I asked Lauter how long one would have to stand in the room before getting a repeat; he answered that it would never happen. There are some 43,000 images, constantly rearranged by a computer. Museum director Ammann has said of Mr. Viola's environment, "The interlocking of silence, overpowering images, sometimes combined with deafening noises, and pauses (to let the after-images have their effect), is an event which almost everyone finds utterly compelling." It's Lauter's opinion that modern art represents a transition stage, and that the great art of the future will be expressed in conjunction with architecture, with the planning of public spaces, or in other ways which connect it with broader aspects of living. But what about painting? Will there be no more great painters? "Oh, we will always have painters," Lauter says. But it is clear that he considers the major task for the artist of the future to be relating his or her work to the total landscape of life. The museum owes its inception to the city of Frankfurt's purchase of works from the Karl Stroher collection. Stroher had moved the family cosmetics business to the West from Leipzig after 1945. After that, he became interested in encouraging the abstract artists whose work the Nazi regime had banned. Along with Peter Ludwig, the Aachen industrialist who has made his contribution to modern art in Cologne, Stroher became a major collector. It had been his intention that his collection remain in Darmstadt, but the state of Hesse (in which both Darmstadt and Frankfurt lie) failed to make the necessary arrangements to permanently house his collection while he was still living. After Stroher's passing in 1977, the family had to sell much of his collection to pay estate taxes. In 1981, Frankfurt managed to acquire 67 of his works. Stroher's own focus on contemporary art and on collecting groups of works has been continued by the MMK. In its tribute to Stroher, the museum quotes his motto, "Don't stand still, keep moving." The commitment to modern works of art seems to mandate that this museum's collections will indeed not stand still.