In a recent Letter,¹ Gautam Desiraju argued two rather controversial points. Primarily, he proclaimed his discomfort with the term cocrystal (or co-crystal), probably first introduced, and certainly popularized, by the late Margaret Etter:^2,3 he suggests it be abandoned. Within publishing response time, Jack Dunitz rapidly countered with a refutation of his arguments, and a robust defense of the term.⁴ This current communication is concerned with challenging Desiraju's secondary point,¹ made almost en passant, that the term pseudopolymorph should be retained, even if ambiguous, because it has entered common usage. I could not disagree more. It is our duty as scientists to avoid ambiguity, and to be as clear in our expositions as possible, especially when terms are likely to become visible to the general public. This is particularly true in this new century, when scientists are increasingly being accused of obfuscation and misleading statements. In a recent editorial, Robin Rogers stated "it is not clear if [the term pseudopolymorph] conveys the scientific meaning it should. As the number of researchers increases in these important fields, care must be taken to provide the community with unambiguous direction on terminology that provides a ready understanding, but also scientific accuracy".⁵ The purpose of this polemic is to reinforce this comment, examine the unsuitability of the term, and to suggest that it be abandoned.

So, let us examine the meaning of the word pseudopolymorph. Taking Desiraju's own definition, polymorphism is "the phenomenon wherein the same substance exhibits different crystal packing arrangements".⁶ If we enlarge on this definition, polymorphism is the ability of a substance to adopt different internal structures and external forms, in response to different conditions of temperature and/or pressure and/or crystallization. Thus, a polymorph is the crystallographer's equivalent to a molecular isomer. For elements, polymorphism expresses itself as allotropy.⁷ It is central, and unequivocal, to the definition and understanding of polymorphism that different polymorphs have the same chemical and molecular composition, differing only in the way those molecules are packed in the lattice. The prefix pseudo- means false or spurious.⁸ Thus, a pseudopolymorph must be a false, or spurious, polymorph. In other words, it must be a material that apparently is a different crystalline form of a material, but in reality is not. Now, as there should never be any doubt, in this century, about the chemical identity of a material, then it follows that solvates of a compound can never be pseudopolymorphs, as there will never be any doubt as to their chemical identity. To draw a parallel in the art world, we could call a forgery of Van Gogh's "Sunflowers", if we so desired, a pseudo-"Sunflowers"-it would be false or spurious, and is similar enough to the original to be mistaken for it. But, we could not describe a fake "Mona Lisa" as a pseudo-"Sunflowers", despite being false, as there was never any possibility of mistaking one for the other.

So let us be clear. The term pseudopolymorph is now commonly being applied to mean the solvate, or (in the specific case of water) hydrate, of a material. We gain no new understanding by introducing the term "pseudopolymorphs", and indeed it is pedagogically misleading. It has been introduced into the literature, but I believe it should be expunged; editors should insist that it is removed from manuscripts in which it is used prior to publication. The term "solvate" has been around for centuries, is universally understood, and is a perfect descriptor for these materials. Why introduce unnecessary and misleading jargon?