Roald Hoffman in American Scientist:
Some theoreticians in chemistry, myself included, like to think about molecules that do not (yet) exist. I use the simple word “think” purposely, for the design need not use fancy-schmancy, computer-intensive, “first-principles” calculations. We conjure up the chemical future in so many ways—through simple model building, qualitative thinking and ever-more-reliable quantum chemical calculations. Even in dreams, as Henning Hopf of the Technische Universität Braunschweig reminded me, referring to the German chemist Friedrich August Kekulé, who worked out the cyclic structure of the carbon-based molecule benzene in the mid-1800s. Kekulé stated that the structure came to him during a daytime reverie about the ouroboric symbol of a snake biting its own tail.
But why do we try to imagine new molecules? Aren’t there enough molecules already on Earth, be they natural or synthetic? A potpourri of reasons follows.