Fig. 4. Free energy surfaces (FESs) at high pressure and temperature.

Results obtained with first principles simulations. Calculated 1D FESs (a), probability distribution of ion-pair (b), and 2D FES (c). The probability distributions are aligned at the position of the contact ion pair (CIP): $\exp \left(-\left(F\left(r\right)-F_{\mathrm{C}IP}\right)/k_{\mathrm{B}}T\right)$. F is the free energy, T the temperature, r the Na–Cl distance and k_B the Boltzmann constant.