Chill+ Ice & Hydrate Classifier

Classify water molecules as hexagonal ice, cubic ice, interfacial ice, clathrate hydrate, interfacial hydrate or liquid with the CHILL+ algorithm.

CHILL+ tells ice and clathrate hydrates apart from liquid water using the correlation of local bond-orientational order between neighboring molecules. Use it to follow ice nucleation and growth, stacking disorder (cubic versus hexagonal ice) and hydrate formation in water simulations.

Key	Version	Builds
`chill-plus`	1.1.1	Linux, macOS, Windows, browser

How it works

Every particle in the frame is treated as a water molecule. For each one, the plugin computes the complex bond-order vector q₃ₘ (m = −3…3) from the spherical harmonics of the bonds to the neighbors within the cutoff. For each bond it then takes the normalized correlation c(i,j) of the two molecules' q₃ vectors:

Staggered bond: c(i,j) < −0.8
Eclipsed bond: −0.35 < c(i,j) < 0.25

A molecule with exactly four neighbors is classified from its bond counts:

Bonds	Structure
4 eclipsed	`HYDRATE`
3 eclipsed	`INTERFACIAL_HYDRATE`
4 staggered	`CUBIC_ICE`
3 staggered and 1 eclipsed	`HEXAGONAL_ICE`
3 staggered and no eclipsed, or 2 staggered	`INTERFACIAL_ICE`

Everything else, including any molecule without exactly four neighbors, is OTHER (liquid).

Inputs

Positions of one particle per water molecule: the oxygen atoms, or the sites of a coarse-grained model such as mW. The plugin doesn't filter by type, so a frame that also holds hydrogens or guest molecules gives wrong neighbor counts.

Arguments

Argument	Type	Default	Allowed values	Description
O-O Cutoff (Å) `cutoff`	number	`3.5`	2 to 10, step 0.1	Neighbor cutoff between water molecules. 3.5 Å sits in the first minimum of the O–O radial distribution function of water.

Outputs

Exposure	What you get
Chill+ Structure	A listing, and a 3D model of the molecules colored by structure.
Chill+ Summary	The frame's totals: atoms, the cutoff, and molecules per structure. Stored with the analysis. No listing.

The workflow declares one per-atom property, structure_type (Structure Type), with the values 0 OTHER, 1 HEXAGONAL_ICE, 2 CUBIC_ICE, 3 INTERFACIAL_ICE, 4 HYDRATE, 5 INTERFACIAL_HYDRATE.

Reference

A. H. Nguyen, V. Molinero, "Identification of clathrate hydrates, hexagonal ice, cubic ice, and liquid water in simulations: the CHILL+ algorithm", J. Phys. Chem. B 119, 9369 (2015).