This keyword is only relevant for NEP3 and NEP4 (see version keyword). It sets the number of basis functions that are used to build the radial and angular descriptor functions, see Sects. II.B and II.C as well as Eq. (3) in [Fan2022b]. The syntax is:

basis_size <N_bas_R> <N_bas_A>

where <N_bas_R> and <N_bas_A> set \(N_\mathrm{bas}^\mathrm{R}\) and \(N_\mathrm{bas}^\mathrm{A}\), respectively. The parameters must satisfy \(0 \leq N_\mathrm{bas}^\mathrm{R},N_\mathrm{bas}^\mathrm{A} \leq 19\).

The default values of \(N_\mathrm{bas}^\mathrm{R}=8\) and \(N_\mathrm{bas}^\mathrm{A}=8\) are usually sufficient.

Note: These parameters should not be confused with \(n_\mathrm{max}^\mathrm{R}\) and \(n_\mathrm{max}^\mathrm{A}\), which are set via the n_max keyword.