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F3/F4 Order Parameters

F3 and F4 are tetrahedral order parameters that quantify the local structure of water molecules, helping distinguish between liquid water, ice, and hydrate phases.

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F3 Parameter

F3 measures the deviation from perfect tetrahedral coordination: F3=∑j,k(∣cos⁡θjik∣⋅cos⁡θjik+19)2F_3 = \sum_{j,k} \left( |\cos\theta_{jik}| \cdot \cos\theta_{jik} + \frac{1}{9} \right)^2 Where θ is the angle between O-O vectors from a central oxygen to its neighbors.
PhaseTypical F3 Value
Liquid water~0.15
Ice~0.0
Hydrate~0.05

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F4 Parameter

F4 uses dihedral angles to detect ordered arrangements: F4=⟨cos⁡(3ϕ)⟩F_4 = \langle \cos(3\phi) \rangle Where φ is the dihedral angle H-O…O-H.
PhaseTypical F4 Value
Liquid water~0.0
Ice Ih~-0.04
Hydrate~0.7

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Running F3/F4 Analysis

  1. Upload your .gro file
  2. Navigate to the ANALYZE tab
  3. Find the F3/F4 ORDER PARAMETERS section
  4. Adjust the O-O cutoff if needed (default: 3.5 Å)
  5. Click ANALYZE ALL or SELECTED ONLY

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Interpreting Results

  • High F4 (~0.7): Strong hydrate-like ordering
  • Low F4 (~0): Liquid-like disorder
  • Negative F4 (~-0.04): Ice-like structure
Use F3/F4 to track hydrate formation over a trajectory.