source: sasmodels/sasmodels/models/core_shell_parallelepiped.py @ 393facf

r"""
Definition
----------

Calculates the form factor for a rectangular solid with a core-shell structure.
The thickness and the scattering length density of the shell or
"rim" can be different on each (pair) of faces.


The form factor is normalized by the particle volume $V$ such that

.. math::

    I(q) = \text{scale}\frac{\langle f^2 \rangle}{V} + \text{background}

where $\langle \ldots \rangle$ is an average over all possible orientations
of the rectangular solid.


The function calculated is the form factor of the rectangular solid below.
The core of the solid is defined by the dimensions $A$, $B$, $C$ such that
$A < B < C$.

.. image:: img/core_shell_parallelepiped_geometry.jpg

There are rectangular "slabs" of thickness $t_A$ that add to the $A$ dimension
(on the $BC$ faces). There are similar slabs on the $AC$ $(=t_B)$ and $AB$
$(=t_C)$ faces. The projection in the $AB$ plane is then

.. image:: img/core_shell_parallelepiped_projection.jpg

The volume of the solid is

.. math::

    V = ABC + 2t_ABC + 2t_BAC + 2t_CAB

**meaning that there are "gaps" at the corners of the solid.**

The intensity calculated follows the :ref:`parallelepiped` model, with the
core-shell intensity being calculated as the square of the sum of the
amplitudes of the core and the slabs on the edges.

the scattering amplitude is computed for a particular orientation of the core-shell
parallelepiped with respect to the scattering vector and then averaged over all
possible orientations, where $\alpha$ is the angle between the $z$ axis and the longest axis $C$
of the parallelepiped, $\beta$ is the angle between projection of the particle in the $xy$ detector plane and the $y$ axis.

.. math::
    \begin{align*}
    F(Q)&=A B C (\rho_\text{core}-\rho_\text{solvent})  S(A \sin\alpha \sin\beta)S(B \sin\alpha \cos\beta)S(C \cos\alpha) \\
    &+ 2t_A B C (\rho_\text{A}-\rho_\text{solvent})  \left[S((A+t_A) \sin\alpha \sin\beta)-S(A \sin\alpha \sin\beta)\right] S(B \sin\alpha \cos\beta) S(C \cos\alpha)\\
    &+ 2 A t_B C (\rho_\text{B}-\rho_\text{solvent})  S(A \sin\alpha \sin\beta) \left[S((B+t_B) \sin\alpha \cos\beta)-S(B \sin\alpha \cos\beta)\right] S(C \cos\alpha)\\
    &+ 2 A B t_C (\rho_\text{C}-\rho_\text{solvent}) S(A \sin\alpha \sin\beta) S(B \sin\alpha \cos\beta) \left[S((C+t_C) \cos\alpha)-S(C \cos\alpha)\right]
    \end{align*}

with

.. math::

    S(x) = \frac{\sin \tfrac{1}{2}Q x}{\tfrac{1}{2}Q x}

where $\rho_\text{core}$, $\rho_\text{A}$, $\rho_\text{B}$ and $\rho_\text{C}$ are
the scattering length of the parallelepiped core, and the rectangular slabs of
thickness $t_A$, $t_B$ and $t_C$, respectively.
$\rho_\text{solvent}$ is the scattering length of the solvent.

FITTING NOTES
If the scale is set equal to the particle volume fraction, $\phi$, the returned
value is the scattered intensity per unit volume, $I(q) = \phi P(q)$.
However, **no interparticle interference effects are included in this
calculation.**

There are many parameters in this model. Hold as many fixed as possible with
known values, or you will certainly end up at a solution that is unphysical.

The returned value is in units of |cm^-1|, on absolute scale.

NB: The 2nd virial coefficient of the core_shell_parallelepiped is calculated
based on the the averaged effective radius $(=\sqrt{(A+2t_A)(B+2t_B)/\pi})$
and length $(C+2t_C)$ values, after appropriately
sorting the three dimensions to give an oblate or prolate particle, to give an
effective radius, for $S(Q)$ when $P(Q) * S(Q)$ is applied.

For 2d data the orientation of the particle is required, described using
angles $\theta$, $\phi$ and $\Psi$ as in the diagrams below, for further details
of the calculation and angular dispersions see :ref:`orientation` .
The angle $\Psi$ is the rotational angle around the *long_c* axis. For example,
$\Psi = 0$ when the *short_b* axis is parallel to the *x*-axis of the detector.

For 2d, constraints must be applied during fitting to ensure that the inequality
$A < B < C$ is not violated, and hence the correct definition of angles is preserved. The calculation will not report an error,
but the results may be not correct.

.. figure:: img/parallelepiped_angle_definition.png

    Definition of the angles for oriented core-shell parallelepipeds.
    Note that rotation $\theta$, initially in the $xz$ plane, is carried out first, then
    rotation $\phi$ about the $z$ axis, finally rotation $\Psi$ is now around the axis of the parallelepiped.
    The neutron or X-ray beam is along the $z$ axis.

.. figure:: img/parallelepiped_angle_projection.png

    Examples of the angles for oriented core-shell parallelepipeds against the
    detector plane.

References
----------

.. [#] P Mittelbach and G Porod, *Acta Physica Austriaca*, 14 (1961) 185-211
    Equations (1), (13-14). (in German)
.. [#] D Singh (2009). *Small angle scattering studies of self assembly in
   lipid mixtures*, Johns Hopkins University Thesis (2009) 223-225. `Available
   from Proquest <http://search.proquest.com/docview/304915826?accountid
   =26379>`_

Authorship and Verification
----------------------------

* **Author:** NIST IGOR/DANSE **Date:** pre 2010
* **Converted to sasmodels by:** Miguel Gonzales **Date:** February 26, 2016
* **Last Modified by:** Wojciech Potrzebowski **Date:** January 11, 2017
* **Currently Under review by:** Paul Butler
"""

import numpy as np
from numpy import pi, inf, sqrt, cos, sin

name = "core_shell_parallelepiped"
title = "Rectangular solid with a core-shell structure."
description = """
     P(q)=
"""
category = "shape:parallelepiped"

#             ["name", "units", default, [lower, upper], "type","description"],
parameters = [["sld_core", "1e-6/Ang^2", 1, [-inf, inf], "sld",
               "Parallelepiped core scattering length density"],
              ["sld_a", "1e-6/Ang^2", 2, [-inf, inf], "sld",
               "Parallelepiped A rim scattering length density"],
              ["sld_b", "1e-6/Ang^2", 4, [-inf, inf], "sld",
               "Parallelepiped B rim scattering length density"],
              ["sld_c", "1e-6/Ang^2", 2, [-inf, inf], "sld",
               "Parallelepiped C rim scattering length density"],
              ["sld_solvent", "1e-6/Ang^2", 6, [-inf, inf], "sld",
               "Solvent scattering length density"],
              ["length_a", "Ang", 35, [0, inf], "volume",
               "Shorter side of the parallelepiped"],
              ["length_b", "Ang", 75, [0, inf], "volume",
               "Second side of the parallelepiped"],
              ["length_c", "Ang", 400, [0, inf], "volume",
               "Larger side of the parallelepiped"],
              ["thick_rim_a", "Ang", 10, [0, inf], "volume",
               "Thickness of A rim"],
              ["thick_rim_b", "Ang", 10, [0, inf], "volume",
               "Thickness of B rim"],
              ["thick_rim_c", "Ang", 10, [0, inf], "volume",
               "Thickness of C rim"],
              ["theta", "degrees", 0, [-360, 360], "orientation",
               "c axis to beam angle"],
              ["phi", "degrees", 0, [-360, 360], "orientation",
               "rotation about beam"],
              ["psi", "degrees", 0, [-360, 360], "orientation",
               "rotation about c axis"],
             ]

source = ["lib/gauss76.c", "core_shell_parallelepiped.c"]


def ER(length_a, length_b, length_c, thick_rim_a, thick_rim_b, thick_rim_c):
    """
        Return equivalent radius (ER)
    """

    # surface average radius (rough approximation)
    surf_rad = sqrt((length_a + 2.0*thick_rim_a) * (length_b + 2.0*thick_rim_b) / pi)

    height = length_c + 2.0*thick_rim_c

    ddd = 0.75 * surf_rad * (2 * surf_rad * height + (height + surf_rad) * (height + pi * surf_rad))
    return 0.5 * (ddd) ** (1. / 3.)

# VR defaults to 1.0

def random():
    import numpy as np
    outer = 10**np.random.uniform(1, 4.7, size=3)
    thick = np.random.beta(0.5, 0.5, size=3)*(outer-2) + 1
    length = outer - thick
    pars = dict(
        length_a=length[0],
        length_b=length[1],
        length_c=length[2],
        thick_rim_a=thick[0],
        thick_rim_b=thick[1],
        thick_rim_c=thick[2],
    )
    return pars

# parameters for demo
demo = dict(scale=1, background=0.0,
            sld_core=1, sld_a=2, sld_b=4, sld_c=2, sld_solvent=6,
            length_a=35, length_b=75, length_c=400,
            thick_rim_a=10, thick_rim_b=10, thick_rim_c=10,
            theta=0, phi=0, psi=0,
            length_a_pd=0.1, length_a_pd_n=1,
            length_b_pd=0.1, length_b_pd_n=1,
            length_c_pd=0.1, length_c_pd_n=1,
            thick_rim_a_pd=0.1, thick_rim_a_pd_n=1,
            thick_rim_b_pd=0.1, thick_rim_b_pd_n=1,
            thick_rim_c_pd=0.1, thick_rim_c_pd_n=1,
            theta_pd=10, theta_pd_n=1,
            phi_pd=10, phi_pd_n=1,
            psi_pd=10, psi_pd_n=1)

# rkh 7/4/17 add random unit test for 2d, note make all params different, 2d values not tested against other codes or models
qx, qy = 0.2 * cos(pi/6.), 0.2 * sin(pi/6.)
tests = [[{}, 0.2, 0.533149288477],
         [{}, [0.2], [0.533149288477]],
         [{'theta':10.0, 'phi':20.0}, (qx, qy), 0.0853299803222],
         [{'theta':10.0, 'phi':20.0}, [(qx, qy)], [0.0853299803222]],
        ]
del qx, qy  # not necessary to delete, but cleaner