Splined radial integrals¶

Classes for generating splines which can be used as tabulated radial integrals in the various SOAP and LODE calculators.

All classes are based on rascaline.utils.RadialIntegralSplinerBase. We provides several ways to compute a radial integral: you may chose and initialize a pre defined atomic density and radial basis and provide them to rascaline.utils.SoapSpliner or rascaline.utils.LodeSpliner. Both classes require scipy to be installed in order to perform the numerical integrals.

Alternatively, you can also explicitly provide functions for the radial integral and its derivative and passing them to rascaline.utils.RadialIntegralFromFunction.

class rascaline.utils.RadialIntegralSplinerBase(max_radial: int, max_angular: int, spline_cutoff: float, basis: RadialBasisBase | None, accuracy: float)¶

Bases: ABC

Base class for splining arbitrary radial integrals.

If RadialIntegralSplinerBase.radial_integral_derivative() is not implemented in a child class it will computed based on finite differences.

Parameters:

max_angular – number of radial components
max_radial – number of angular components
spline_cutoff – cutoff radius for the spline interpolation. This is also the maximal value that can be interpolated.
basis – Provide a RadialBasisBase instance to orthonormalize the radial integral.
accuracy – accuracy of the numerical integration and the splining. Accuracy is reached when either the mean absolute error or the mean relative error gets below the accuracy threshold.

compute(n_spline_points: int | None = None) → Dict¶

Compute the spline for rascaline’s tabulated radial integrals.

Parameters:: n_spline_points – Use fixed number of spline points instead of find the number based on the provided accuracy.
Returns dict:: dictionary for the input as the radial_basis parameter of a rascaline calculator.

abstract radial_integral(n: int, ell: int, positions: ndarray) → ndarray¶: evaluate the radial integral

property center_contribution: None | ndarray¶

Pre-computed value for the contribution of the central atom.

Required for LODE calculations. The central atom contribution will be orthonormalized in the same way as the radial integral.

radial_integral_derivative(n: int, ell: int, positions: ndarray) → ndarray¶: evaluate the derivative of the radial integral

class rascaline.utils.SoapSpliner(cutoff: float, max_radial: int, max_angular: int, basis: RadialBasisBase, density: AtomicDensityBase, accuracy: float = 1e-08)¶

Bases: RadialIntegralSplinerBase

Compute radial integral spline points for real space calculators.

Use only in combination with a real space calculators like rascaline.SphericalExpansion or rascaline.SoapPowerSpectrum. For k-space spherical expansions use LodeSpliner.

If density is either rascaline.utils.DeltaDensity or rascaline.utils.GaussianDensity the radial integral will be partly solved analytical. These simpler expressions result in a faster and more stable evaluation.

Parameters:

cutoff – spherical cutoff for the radial basis
max_radial – number of angular components
max_angular – number of radial components
basis – definition of the radial basis
density – definition of the atomic density
accuracy – accuracy of the numerical integration and the splining. Accuracy is reached when either the mean absolute error or the mean relative error gets below the accuracy threshold.

Raises:

ValueError – if scipy is not available

Example¶

First import the necessary classed and define hyper parameters for the spherical expansions.

>>> from rascaline import SphericalExpansion
>>> from rascaline.utils import GaussianDensity, GtoBasis

>>> cutoff = 2
>>> max_radial = 6
>>> max_angular = 4
>>> atomic_gaussian_width = 1.0

Next we initialize our radial basis and the density

>>> basis = GtoBasis(cutoff=cutoff, max_radial=max_radial)
>>> density = GaussianDensity(atomic_gaussian_width=atomic_gaussian_width)

And finally the actual spliner instance

>>> spliner = SoapSpliner(
...     cutoff=cutoff,
...     max_radial=max_radial,
...     max_angular=max_angular,
...     basis=basis,
...     density=density,
...     accuracy=1e-3,
... )

Above we reduced accuracy from the default value of 1e-8 to 1e-3 to speed up calculations.

As for all spliner classes you can use the output RadialIntegralSplinerBase.compute() method directly as the radial_basis parameter.

>>> calculator = SphericalExpansion(
...     cutoff=cutoff,
...     max_radial=max_radial,
...     max_angular=max_angular,
...     center_atom_weight=1.0,
...     atomic_gaussian_width=atomic_gaussian_width,
...     radial_basis=spliner.compute(),
...     cutoff_function={"Step": {}},
... )

You can now use calculator to obtain the spherical expansion coefficients of your systems. Note that the the spliner based used here will produce the same coefficients as if radial_basis={"Gto": {}} would be used.

An additional example using a “rectangular” Laplacian eigenstate (LE) basis is provided in the Laplacian eigenstate basis.

Example¶

First import the necessary classed and define hyper parameters for the spherical expansions.

>>> from rascaline import LodeSphericalExpansion
>>> from rascaline.utils import GaussianDensity, GtoBasis

Note that cutoff defined below denotes the maximal distance for the projection of the density. In contrast to SOAP, LODE also takes atoms outside of this cutoff into account for the density.

>>> cutoff = 2
>>> max_radial = 6
>>> max_angular = 4
>>> atomic_gaussian_width = 1.0

\(1.2 \, \pi \, \sigma\) where \(\sigma\) is the atomic_gaussian_width which is a reasonable value for most systems.

>>> k_cutoff = 1.2 * np.pi / atomic_gaussian_width

Next we initialize our radial basis and the density

>>> basis = GtoBasis(cutoff=cutoff, max_radial=max_radial)
>>> density = GaussianDensity(atomic_gaussian_width=atomic_gaussian_width)

And finally the actual spliner instance

>>> spliner = LodeSpliner(
...     k_cutoff=k_cutoff,
...     max_radial=max_radial,
...     max_angular=max_angular,
...     basis=basis,
...     density=density,
... )

As for all spliner classes you can use the output RadialIntegralSplinerBase.compute() method directly as the radial_basis parameter.

>>> calculator = LodeSphericalExpansion(
...     cutoff=cutoff,
...     max_radial=max_radial,
...     max_angular=max_angular,
...     center_atom_weight=1.0,
...     atomic_gaussian_width=atomic_gaussian_width,
...     potential_exponent=1,
...     radial_basis=spliner.compute(),
... )

Example¶

First define a radial_integral function

>>> def radial_integral(n, ell, r):
...     return np.sin(r)
...

and provide this as input to the spline generator

>>> spliner = RadialIntegralFromFunction(
...     radial_integral=radial_integral,
...     max_radial=12,
...     max_angular=9,
...     spline_cutoff=8.0,
... )

Finally, we can use the spliner directly in the radial_integral section of a calculator

>>> from rascaline import SoapPowerSpectrum
>>> calculator = SoapPowerSpectrum(
...     cutoff=8.0,
...     max_radial=12,
...     max_angular=9,
...     center_atom_weight=1.0,
...     radial_basis=spliner.compute(),
...     atomic_gaussian_width=1.0,  # ignored
...     cutoff_function={"Step": {}},
... )

The atomic_gaussian_width parameter is required by the calculator but will be will be ignored during the feature computation.

A more in depth example using a “rectangular” Laplacian eigenstate (LE) basis is provided in the Splined radial integral how-to guide.

radial_integral(n: int, ell: int, positions: ndarray) → ndarray¶: evaluate the radial integral

property center_contribution: None | ndarray¶

Pre-computed value for the contribution of the central atom.

Required for LODE calculations. The central atom contribution will be orthonormalized in the same way as the radial integral.

radial_integral_derivative(n: int, ell: int, positions: ndarray) → ndarray¶: evaluate the derivative of the radial integral