In terms, molecular species (LH:2), from molecular, meaning "like a molecule", + species meaning "entity that reproduces with its own kind" refers to all participants in a reaction mechanism (Grunwald, 1997); a near-synonym is: chemical species.
In 1875, Grant Allen, in his Force and Energy: a Theory of Dynamics, building on the work of James Maxwell, Peter Tait, Balfour Stewart, and Hermann Helmholtz, was employing the term “molecular species”, in respect to the potential energy of a cannon ball, when dropped from a height, transforming into kinetic energy during its fall, then into impact energy of the “molecular species” when hitting the ground.
In late 1910s, Howard Lovecraft, in his correspondence letters, was alluding to the premise that, from the advanced perspective, humans were a form of atomic or molecular species; the following two quotes outline this view:
- “How do we know that that form of atomic and molecular motion we call ‘life’ is the highest of all forms? Perhaps the dominate creature — the most rational and god-like of all beings — is an ‘invisible gas’ Or perhaps it is a flaming and effulgent mass of molten stardust. Who can say that men have souls while rocks have none?”
- — Howard Lovecraft (1916), “Letter to Rheinhart Kleiner, Ira Cole, and Maurice Moe”, Aug 8 
- “My attitude has always been cosmic, and I looked on man as if from another planet. He was merely an interesting species presented for study and classification.”
- — Howard Lovecraft (1922), “A Confession of Unfaith” 
In 1997, Ernest Grunwald, in his Thermodynamics of Molecular Species, building on Lewis (1923) outlined some of the basics of chemical thermodynamics applied to "molecular species" which he defined as all the "participants involved in reaction mechanism". Said another way:
- “Molecular species are macroscopic or near-macroscopic ensembles of molecules that represent, in the real world, the molecules symbolized in chemical equations.”
- — Ernest Grunwald (1997), Thermodynamics of Molecular Species (pg. xiii)
Grunwald describes a three-level scheme for speaking about molecular species:
|1.||List of the formal components of the system|
|2.||List of the molecular species that stem from the components|
|3.||List of any subspecies derived from the molecular species|
Moreover, only those molecular species that are chemically or stoichiometrically significant are listed. This listing or model, according to Grunwald can be changed as our understanding of the chemistry of the system improves.
Chemical | Molecular species
Presently, there does not seem to be any agreed upon consensus as to the distinction between "chemical species", which derives from the Egyptian term keme meaning "black soil", and "molecular species", which derives from the French mole meaning "mass".
The following are quotes:
- “It turns out that the traditional chemical thermodynamics, which deals with processes among equilibrium states, is incomplete because it takes the existence and maintenance of equilibrium for granted and fails to specify it explicitly. This is a surprising omission in a science that prides itself on the precision of its specifications. By contrast, the thermodynamics of molecular species includes departures from equilibrium as a matter of course, and the maintenance of equilibrium is specified explicitly. The specifying mathematics then adds specific terms — I call them molar-shift terms — that are missing from the traditional formulations.”
- — Ernest Grunwald (1997), Thermodynamics of Molecular Species (pg. xiv) 
- Merriam-Webster Collegiate Dictionary, 2000.
- Allen, Grant. (1875). Force and Energy: a Theory of Dynamics (pg. 17). Humboldt, 1889.
- Lovecraft, Howard P. (2010). Against Religion: the Atheistic Writings of H.P. Lovecraft (editor: S.T. Joshi; foreword: Christopher Hitchens) (abs) (Amz) (rocks, pg. 9). Sporting Gentlemen.
- (a) Lovecraft, Howard P. (1922). “A Confession of Unfaith”, Liberal, Feb.
(b) Lovecraft, Howard P. (2010). Against Religion: the Atheistic Writings of H.P. Lovecraft (editor: Sunand Joshi; foreword: Christopher Hitchens) (abs) (Amz) (pg. 5). Sporting Gentlemen.
- Ernest Grunwald – Hmolpedia 2020.
- Grunwald, Ernest. (1997). Thermodynamics of Molecular Species (Amz). Wiley.
- Dickerson, Richard E. (1969). Molecular Thermodynamics (molecular species, 3+ pgs). Benjamin.