pyscfad.pbc.gto.cell.Cell.get_ewald_params

Cell.get_ewald_params(precision=None, mesh=None)

Choose a reasonable value of Ewald ‘eta’ and ‘cut’ parameters. eta^2 is the exponent coefficient of the model Gaussian charge for nucleus at R: frac{eta^3}{pi^1.5} e^{-eta^2 (r-R)^2}

Choice is based on largest G vector and desired relative precision.

The relative error in the G-space sum is given by

precision ~ 4pi Gmax^2 e^{(-Gmax^2)/(4 eta^2)}

which determines eta. Then, real-space cutoff is determined by (exp. factors only)

precision ~ erfc(eta*rcut) / rcut ~ e^{(-eta**2 rcut*2)}

Returns:

ew_eta, ew_cutfloat

The Ewald ‘eta’ and ‘cut’ parameters.