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Liquid water can be cooled below 0 degrees Celsius without immediately freezing because the formation of ice crystals requires nucleation. This made me think about the reverse situation: can ice nucleation occur above 0 degrees Celsius? It seems that there should be a statistical chance of water molecules aligning to create an ice crystal (even if that crystal is thermodynamically unstable and quickly returns to the liquid state). Is there a numerical way of describing the odds of nucleation sites forming at any given temperature?

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  • $\begingroup$ Such temporary, molecular level scope changes occur in liquid water regularly for t < 3.98 °C. $\endgroup$
    – Poutnik
    Jul 26, 2022 at 13:00
  • $\begingroup$ Is it still theoretically possible above 3.98 °C, just increasingly uncommon due to thermodynamic and kinetic barriers? $\endgroup$
    – Akash
    Jul 26, 2022 at 13:20
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    $\begingroup$ Yes you'll get density fluctuations that may look ice-like, but since there is no thermodynamic driving force there will be no critical nucleus size and no crystallization above $T= 0^{\circ}\mathrm{C}$ at $P = 1\mathrm{bar}$. $\endgroup$
    – Hayden S
    Jul 26, 2022 at 15:47
  • $\begingroup$ So a few molecules might arrange themselves into an ice crystal structure, but it can never go beyond that? $\endgroup$
    – Akash
    Jul 26, 2022 at 17:14

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Yes, there are several numerical (computer simulation) ways to quantify the odds of formation of nucleation sites. One of them is a modern molecular sampling technique called Metadynamics (For eg: See https://doi.org/10.1107/S2052252514027626). Here, the free energy of the nucleation process can be estimated along a strategically chosen crystallization pathway or "collective variable". The likelihood of crossing barriers along this free energy curve can be used to estimate the frequency of nucleation events. I believe the nucleation barrier would be much greater than thermal energy (kT) at 1 bar and T >273 K leading to very large barrier crossing time.. (that is never..)

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