Requests for technical support from the VASP group should be posted in the VASP-forum.

# ML MRB2

ML_MRB2 = [integer]
Default: ML_MRB2 = ML_MRB1

Description: This tag sets the number $N_{\text{R}}^{l}$ (for all $l$ ) of radial basis functions used to expand the angular descriptor within the machine learning force field method.

The angular descriptor is constructed from

$\rho _{i}^{(3)}\left(r,s,\theta \right)=\iint d{\hat {\mathbf {r} }}d{\hat {\mathbf {s} }}\delta \left({\hat {\mathbf {r} }}\cdot {\hat {\mathbf {s} }}-\mathrm {cos} \theta \right)\sum \limits _{j=1}^{N_{a}}\sum \limits _{k\neq j}^{N_{a}}\rho _{ik}\left(r{\hat {\mathbf {r} }}\right)\rho _{ij}\left(s{\hat {\mathbf {s} }}\right),\quad {\text{where}}\quad \rho _{ij}\left(\mathbf {r} \right)=f_{\mathrm {cut} }\left(r_{ij}\right)g\left(\mathbf {r} -\mathbf {r} _{ij}\right)$ and $g\left(\mathbf {r} \right)$ is an approximation of the delta function. In practice, the continuous function above is transformed into a discrete set of numbers $p_{n\nu l}^{i}$ by expanding it into a set of radial basis functions $\chi _{nl}(r)$ and Legendre polynomials $P_{l}\left(\mathrm {cos} \theta \right)$ (see this section for more details):

$\rho _{i}^{(3)}\left(r,s,\theta \right)=\sum \limits _{l=1}^{L_{\mathrm {max} }}\sum \limits _{n=1}^{N_{\mathrm {R} }^{l}}\sum \limits _{\nu =1}^{N_{\mathrm {R} }^{l}}{\sqrt {\frac {2l+1}{2}}}p_{n\nu l}^{i}\chi _{nl}\left(r\right)\chi _{\nu l}\left(s\right)P_{l}\left(\mathrm {cos} \theta \right).$ The tag ML_MRB2 sets the number $N_{\text{R}}^{l}$ of radial basis functions to use in this expansion. The same number is used for all $l$ .

 Mind: The number of angular descriptor expansion coefficients $p_{n\nu l}^{i}$ scales quadratically with $N_{\text{R}}^{l}$ set by this tag. It also depends on ML_LMAX2 and the number of elements.