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# 4.5. Accessing per-atom data

This page discusses how per-atom data is managed in LAMMPS, how it can be accessed, what communication patterns apply, and some of the utility functions that exist for a variety of purposes.

## 4.5.1. Owned and ghost atoms

As described on the parallel partitioning algorithms page, LAMMPS uses a domain decomposition of the simulation domain, either in a brick or tiled manner. Each MPI process owns exactly one subdomain and the atoms within it. To compute forces for tuples of atoms that are spread across sub-domain boundaries, also a “halo” of ghost atoms are maintained within the communication cutoff distance of its subdomain.

The total number of atoms is stored in Atom::natoms (within any typical class this can be referred to at atom->natoms). The number of owned (or “local” atoms) are stored in Atom::nlocal; the number of ghost atoms is stored in Atom::nghost. The sum of Atom::nlocal over all MPI processes should be Atom::natoms. This is by default regularly checked by the Thermo class, and if the sum does not match, LAMMPS stops with a “lost atoms” error. For convenience also the property Atom::nmax is available, this is the maximum of Atom::nlocal + Atom::nghost across all MPI processes.

Per-atom properties are either managed by the atom style, individual classes, or as custom arrays by the individual classes. If only access to owned atoms is needed, they are usually allocated to be of size Atom::nlocal, otherwise of size Atom::nmax. Please note that not all per-atom properties are available or updated on ghost atoms. For example, per-atom velocities are only updated with comm_modify vel yes.

## 4.5.2. Atom indexing

When referring to individual atoms, they may be indexed by their local index, their index in their Atom::x array. This is densely populated containing first all owned atoms (index < Atom::nlocal) and then all ghost atoms. The order of atoms in these arrays can change due to atoms migrating between between subdomains, atoms being added or deleted, or atoms being sorted for better cache efficiency. Atoms are globally uniquely identified by their atom ID. There may be multiple atoms with the same atom ID present, but only one of them may be an owned atom.

To find the local index of an atom, when the atom ID is known, the Atom::map() function may be used. It will return the local atom index or -1. If the returned value is between 0 (inclusive) and Atom::nlocal (exclusive) it is an owned or “local” atom; for larger values the atom is present as a ghost atom; for a value of -1, the atom is not present on the current subdomain at all.

If multiple atoms with the same tag exist in the same subdomain, they can be found via the Atom::sametag array. It points to the next atom index with the same tag or -1 if there are no more atoms with the same tag. The list will be exhaustive when starting with an index of an owned atom, since the atom IDs are unique, so there can only be one such atom. Example code to count atoms with same atom ID in a subdomain:

    for (int i = 0; i < atom->nlocal; ++i) {
      int count = 0;
      while (sametag[i] >= 0) {
        i = sametag[i];
        ++count;
      }
      printf("Atom ID: %ld is present %d times\n", atom->tag[i], count);
    }

## 4.5.3. Atom class versus AtomVec classes

The Atom class contains all kinds of flags and counters about atoms in the system and that includes pointers to all per-atom properties available for atoms. However, only a subset of these pointers are non-NULL and which those are depends on the atom style. For each atom style there is a corresponding AtomVecXXX class derived from the AtomVec base class, where the XXX indicates the atom style. This AtomVecXXX class will update the counters and per-atom pointers if atoms are added or removed to the system or migrate between subdomains.