# SOAP and GAP reference

This page collects useful information in working with the GAP code.

## SOAP vectors

The description of the local environment by SOAP vectors is as follows. The neighbour density around each atom is expanded in a set of radial and angular basis functions, with a different expansion for each atomic species in the neighbour environment. The SOAP vector is then formed from these expansion coefficients. The array has two radial and one angular indices. The number of radial basis functions is n_max, the number of angular basis functions l_max+1. The order of the elements in the vector is built as follows.

```
for i=1:n_species*n_max
for j=i:n_species*n_max
for l=0:l_max
```