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Analysis

Projects particle-carried quantities onto a regular analysis grid — the grid used for parallelism, subdivided by grid_subdiv — for later output or connected-component analysis.

atom_cell_projection

Syntax
atom_cell_projection:
  fields: [<string>, ...]
  grid_subdiv: <int>
  splat_size: <float>
Parameters
fields:        list of strings, default [".*"]  # Regular expressions selecting which quantities to project — count, velocity, force, vnorm, mv2, mass, momentum, mv2tensor.
grid_subdiv:   int, default 1                   # Per-cell subdivision of the projection grid.
splat_size:    float, default 1.0               # Distance used to spread each particle's contribution onto neighboring grid cells.

Projects per-particle quantities onto a regular grid (grid_cell_values) by splatting each particle's contribution across nearby (sub)cells within splat_size. Requires resize_grid_cell_values to have run first (see Setters) and a species block to already be declared.

Usage example
- grid_flavor
- resize_grid_cell_values
- atom_cell_projection:
    fields: [ mv2, mass, vnorm ]
    grid_subdiv: 2
    splat_size: 1.5 ang

Projecting computed per-particle fields

Besides the built-in quantities, atom_cell_projection also projects the per-particle fields created at run time by other operators (scalar, vector or tensor), for example the mechanical metrics of Particles Features → Analysis (defgrad, green_lagrange, von_mises, ...). Select them by name in fields, like the built-in ones.

particle_cell_projection (exaNBody/src/analytics/) is the generic, application-independent version: it projects count, velocity and force without the species-mass-aware quantities (kinetic energy, momentum) of atom_cell_projection.

If particle positions/velocities are shared across MPI ranks or updated between projections, ghost_update_r_v (and its siblings ghost_update_r, ghost_update_r_v_vir, ghost_update_rq, ghost_update_all, …) syncs the relevant fields into ghost cells first — needed for projection accuracy right at domain/subdomain boundaries:

Usage example
- grid_flavor
- resize_grid_cell_values
- ghost_update_r_v
- atom_cell_projection:
    fields: [ mv2, mass, vnorm ]
    grid_subdiv: 2
    splat_size: 1.5 ang

Not yet covered here

Found in source, out of scope for this page

A handful of other operators also read or write per-cell grid_cell_values data but belong to more specific subsystems rather than core Grids Features, and aren't documented in depth on this page: fluid_friction (a drag-force model driven by a cell-centered velocity field), cc_label (connected-component clustering on a thresholded cell field), grid_stats (a debug/diagnostic operator), and the migrate_cell_particles* family (keeps grid_cell_values consistent across MPI load-balancing events). Flagged here so they aren't mistaken for missing or unknown — they exist, just outside this page's scope for now.

Two-temperature model

The two-temperature model stores the electronic temperature in a te grid field (one value per subcell) and evolves it with init_ttm and ionic_eletronic_heat_transfer. See Two-Temperature Model.