The best replacement for free energy is entropy.

The best alternatives to free energy are rated as following:
★★★★★  entropy
★★★★  activation energy
★★★  standard free energy
★★★  reaction coordinate
★★  internal energy
★  actual free energy
★  low free energy
  helmholtz
  extent of reaction
  equilibrium constant

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Speaking about free energy, one probably shouldn't forget about equilibrium constant.

Other search terms related to free energy and their ratings are:
★★★★★ equilibrium constant
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