If you want to typeset beautiful chemical structures using LaTeX, then you will almost certainly wish to consider using the XyMTeX package. But if you want to concurrently use the fontspec package, because you desire more freedom in selecting fonts for your document, then you may encounter the following error when compiling your document:
LaTeX Error: Command ‘\Gamma’ already defined.
Inspired by a thread in which is similar error is discussed—but instead pertaining to the simultaneous loading of fontspec with some other package—I have discovered that the solution for this problem appears to be a matter of simply loading the fontspec package with the “no-math” option, like thus:
\usepackage[no-math]{fontspec}
