xnn.common.models.electrostatics.ewald_parameters#

xnn.common.models.electrostatics.ewald_parameters(cutoff, accuracy)[source]#

Splitting parameter and reciprocal cutoff for a real-space cutoff and an accuracy.

alpha = sqrt(-ln eps) / r_c and k_c = 2 alpha sqrt(-ln eps): the real-space screening exp(-alpha^2 r_c^2) and the reciprocal Gaussian exp(-k_c^2 / 4 alpha^2) both equal eps at their cutoffs (the Kolafa-Perram balance for a given r_c).

Parameters:

cutoff (float) – Real-space cutoff r_c in Angstrom.
accuracy (float) – Target relative accuracy eps in (0, 1).

Returns:

(alpha, k_cutoff) in 1/Angstrom.

Return type:

tuple of float