However, there are some limitations when choosing the molecule names. Molecule names should be composed of letter, digit characters, and the '_' character. This means that molecule names cannot contain special characters, such as '=', '-', etc. with the exception of '_'. Molecule name definitions in functionalgroups.cxsmi file can contain whitespace characters (space, tab), but when names are referenced from a Chemical Terms expression the whitespace characters should be replaced with a single '_' character (e.g. secondary amine should be referred as secondary_amine in Chemical Terms expressions).