Valid values for fields are "SMILES",
"Formula", "InChI", "InChIKey", "StdInChI",
"StdInChIKey", "AverageMass", "MolecularWeight",
"MonoisotopicMass", "NominalMass", "CommonName",
"ReferenceCount", "DataSourceCount", "PubMedCount",
"RSCCount", "Mol2D", "Mol3D". You can specify any
number of fields.