This keyword is only relevant for NEP3 and NEP4. It sets the maximum expansion order for the angular terms, see Sect. II.C of [Fan2022b]. The syntax is:

l_max <l_max_3b> {<l_max_4b> {<l_max_5b>}}

where l_max_3b, l_max_4b, and l_max_5b set the limits for three, four, and five-body terms, respectively. The latter two arguments are optional (as indicated by the curly brackets).

If there is one value \(l_\mathrm{max}^\mathrm{4b}=l_\mathrm{max}^\mathrm{5b}=0\). If there are two values \(l_\mathrm{max}^\mathrm{5b}=0\).

\(l_\mathrm{max}^\mathrm{3b}\) can only be 4, \(l_\mathrm{max}^\mathrm{4b}\) can be 0 or 2, and \(l_\mathrm{max}^\mathrm{5b}\) can be 0 or 1.

The default values are \(l_\mathrm{max}^\mathrm{3b}=4\), \(l_\mathrm{max}^\mathrm{4b}=2\), and \(l_\mathrm{max}^\mathrm{5b}=0\).