|Excerpt from the manuscript of Freud's 1895 "Project for Scientific Psychology", the highlighted section reads: ‘the intention [of this project] is to furnish a psychology that shall be a natural science’, the natural science referred here said to mean one that studies matter using experimentation, measurement, and formulation. |
“The intention is to furnish a psychology that shall be a natural science; that is, to represent psychical processes as a quantitatively determinate state of specifiable material particles.”
The mention of states of atoms or molecules seems to be a direct usage of "state" as defined in thermodynamics, as being quantified by a specific intensive and extensive variables. Freud then goes on to project that the psychology of the future will be one based on chemistry and energetics or rather thermodynamics, in the modern sense: 
“In the future, psychologists will exercise a direct influence, by means of particular chemical substances, on the amounts of energy and their distribution in the mental apparatus.”
It is said that Austrian philosopher Wilhelm Jerusalem’s 1895 The Function of Judgment greatly inspired Freud’s “Project”. 
The initiation of Freud’s ambitions plans to develop a hard science version of psychology based on thermodynamics and mechanical theory, was first laid out in his “Project for Scientific Psychology”, began in the spring of 1895, but never finished and published post humorously in 1950, in volume one of his twenty-four volume collected works. 
Free energy | Bound energy
It is said that in this is paper Freud gave his first outline of his views on ‘bound energy’ and ‘unbound energy’ (or free energy) in the states of consciousness; terms which had only recently been introduced in Hermann Helmholtz’ 1882 paper “The Thermodynamics of Chemical Processes”, one the founding papers of chemical thermodynamics.
In Freud’s theory of ‘bound’ and ‘unbound’ psychical energy, which itself is an extrapolation of Helmholtz’s interpretation of “bound energy” (TS) and “free energy” (F), according to the views of French psychoanalyst Jean Laplanche, a student of Jacques Lacan, it is said that Freud carried this through into his death instinct theory, viewing unbound energy as death.  In a modern sense, this may be seen as being correct in the sense of the Gibbs free energy minimum of a reaction being seen as the dead state of a reaction, where forward reaction ceases to progress, such as typified by a 'dead relationship'.
1. (a) Freud, Sigmund. (1895). A Project for Scientific Psychology (unfinished manuscript), in The Origins of Psychoanalysis: Letters to Wilhelm Fliess, Drafts, and Notes: 1887-1902 (pgs. 345-445). Basic Books, 1977.
(b) Project for a Scientific Psychology (excerpt) – LOC.gov.
2. Heller, Sharon. (2005). Freud A to Z (Section: Project for Scientific Psychology, pg. 183). Wiley.
3. Geerardyn, Filip and Van de Vivjer, Gertrudis. (2002). The Pre-Psychoanalytic Writings of Sigmund Freud (pg. 12). Karnac Books.
4. Freud, Sigmund. (1895). “A Project for Scientific Psychology” (cited by 1505), in The Standard Edition of the Complete Psychological Works of Sigmund Freud, 1950; Vol. 1: 283-397. London: Hogarth Press, 1966.
5. Lechte, John. (1994). Fifty Key Contemporary Thinkers: from Structuralism to Postmodernity (pg. 21). Routledge.
6. Auyang, Sunny Y. (1999). “Beyond Reductionism in Biology”, Talk presented in the Department of History and Philosophy of Science, University of Sydney, May; CreatingTechnology.org.
● Cohen, Sanford I. (1964). Re-exploring A Project for Scientific Psychology. Self-Published.