I am trying to calculate the Gibbs free energy in an NVT simulation. Although

$$\Delta G = -RT\ln Q,$$

in my simulation I can't calculate

$$Q = \frac{[\text{Products}]}{[\text{Reactants}]}$$

because I cant determine the concentrations of reactants and products.

Hence, I am looking for a relationship between reaction quotient $Q$ and potential energy $U$ as well as $p,$ $V,$ $T,$ i.e.

$$Q(U,p,V,T) = ?$$

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    $\begingroup$ $G$ itself is not equal to $RT \ln Q$; you either want $$\Delta_\mathrm{r}G ^\circ = -RT\ln K,$$ or $$\Delta_\mathrm{r} G = \Delta_\mathrm{r} G^\circ + RT \ln Q.$$ Aside from that, I'm highly skeptical that one can find a relationship between $Q$ and $U$, assuming that by $U$ you mean "the total internal energy of the system". $\endgroup$ – orthocresol Jul 19 '20 at 12:45
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    $\begingroup$ Did you meant "reaction quotient" instead of reaction coordinate? $\endgroup$ – Rahul Verma Jul 19 '20 at 12:52
  • $\begingroup$ @orthocresol I am looking for U as potential energy. Forgive my lack of proficiency with the terms, if total internal energy is essentially the same as potential energy. $\endgroup$ – fireball.1 Jul 19 '20 at 12:56
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    $\begingroup$ Terminology aside, your first equation is still concerning, to me at least. $\endgroup$ – orthocresol Jul 19 '20 at 14:11
  • $\begingroup$ @orthocresol yes you are right w.r.t. the equation. However, since I will be calculating the Gibb's Free energy at each frame, I can calculate the difference without the equilibrium gibbs free energy as that would be the common term in all energies and the end result I am looking for is the difference. $\endgroup$ – fireball.1 Jul 19 '20 at 15:09