source: sasmodels/sasmodels/models/stickyhardsphere.py @ 7e224c2

# Note: model title and parameter table are inserted automatically
r"""This calculates the interparticle structure factor for a hard sphere fluid with
a narrow attractive well. A perturbative solution of the Percus-Yevick closure is used.
The strength of the attractive well is described in terms of "stickiness"
as defined below. The returned value is a dimensionless structure factor, *S(q)*.

The perturb (perturbation parameter), |epsilon|, should be held between 0.01 and 0.1.
It is best to hold the perturbation parameter fixed and let the "stickiness" vary to
adjust the interaction strength. The stickiness, |tau|, is defined in the equation
below and is a function of both the perturbation parameter and the interaction strength.
|tau| and |epsilon| are defined in terms of the hard sphere diameter (|sigma| = 2\*\ *R*\ ),
the width of the square well, |bigdelta| (same units as *R*), and the depth of the well,
*Uo*, in units of kT. From the definition, it is clear that smaller |tau| means stronger
attraction.

.. image:: img/stickyhardsphere_228.PNG

where the interaction potential is

.. image:: img/stickyhardsphere_229.PNG

The Percus-Yevick (PY) closure was used for this calculation, and is an adequate closure
for an attractive interparticle potential. This solution has been compared to Monte Carlo
simulations for a square well fluid, with good agreement.

The true particle volume fraction, |phi|, is not equal to *h*, which appears in most of
the reference. The two are related in equation (24) of the reference. The reference also
describes the relationship between this perturbation solution and the original sticky hard
sphere (or adhesive sphere) model by Baxter.

NB: The calculation can go haywire for certain combinations of the input parameters,
producing unphysical solutions - in this case errors are reported to the command window and
the *S(q)* is set to -1 (so it will disappear on a log-log plot). Use tight bounds to keep
the parameters to values that you know are physical (test them) and keep nudging them until
the optimization does not hit the constraints.

In sasview the effective radius will be calculated from the parameters used in the
form factor P(Q) that this S(Q) is combined with.

For 2D data: The 2D scattering intensity is calculated in the same way as 1D, where
the *q* vector is defined as

.. math::

    Q = \sqrt{Q_x^2 + Q_y^2}

==============  ========  =============
Parameter name  Units     Default value
==============  ========  =============
effect_radius   |Ang|     50
perturb         None      0.05
volfraction     None      0.1
stickiness      K         0.2
==============  ========  =============

.. image:: img/stickyhardsphere_230.jpg

*Figure. 1D plot using the default values (in linear scale).*

REFERENCE

S V G Menon, C Manohar, and K S Rao, *J. Chem. Phys.*, 95(12) (1991) 9186-9190
"""

# TODO: refactor so that we pull in the old sansmodels.c_extensions

from numpy import inf

name = "stickyhardsphere"
title = "Sticky hard sphere structure factor, with Percus-Yevick closure"
description = """\
        [Sticky hard sphere structure factor, with Percus-Yevick closure]
        Interparticle structure factor S(Q)for a hard sphere fluid with
                a narrow attractive well. Fits are prone to deliver non-physical
                parameters, use with care and read the references in the full manual.
                In sasview the effective radius will be calculated from the
                parameters used in P(Q).
"""
category = "structure-factor"

parameters = [
    #   [ "name", "units", default, [lower, upper], "type",
    #     "description" ],
    ["effect_radius", "Ang", 50.0, [0, inf], "volume",
     "effective radius of hard sphere"],
    ["volfraction", "", 0.2, [0, 0.74], "",
     "volume fraction of hard spheres"],
    ["perturb", "", 0.05, [0.01, 0.1], "",
     "perturbation parameter, epsilon"],
    ["stickiness", "", 0.20, [-inf, inf], "",
     "stickiness, tau"],
    ]

# No volume normalization despite having a volume parameter
# This should perhaps be volume normalized?
form_volume = """
    return 1.0;
    """

Iq = """
        double onemineps,eta;
        double sig,aa,etam1,etam1sq,qa,qb,qc,radic;
        double lam,lam2,test,mu,alpha,beta;
        double kk,k2,k3,ds,dc,aq1,aq2,aq3,aq,bq1,bq2,bq3,bq,sq;

        onemineps = 1.0-perturb;
        eta = volfraction/onemineps/onemineps/onemineps;

        sig = 2.0 * effect_radius;
        aa = sig/onemineps;
        etam1 = 1.0 - eta;
        etam1sq=etam1*etam1;
        //C
        //C  SOLVE QUADRATIC FOR LAMBDA
        //C
        qa = eta/12.0;
        qb = -1.0*(stickiness + eta/etam1);
        qc = (1.0 + eta/2.0)/etam1sq;
        radic = qb*qb - 4.0*qa*qc;
        if(radic<0) {
                //if(x>0.01 && x<0.015)
                //      Print "Lambda unphysical - both roots imaginary"
                //endif
                return(-1.0);
        }
        //C   KEEP THE SMALLER ROOT, THE LARGER ONE IS UNPHYSICAL
        lam = (-1.0*qb-sqrt(radic))/(2.0*qa);
        lam2 = (-1.0*qb+sqrt(radic))/(2.0*qa);
        if(lam2<lam) {
                lam = lam2;
        }
        test = 1.0 + 2.0*eta;
        mu = lam*eta*etam1;
        if(mu>test) {
                //if(x>0.01 && x<0.015)
                // Print "Lambda unphysical mu>test"
                //endif
                return(-1.0);
        }
        alpha = (1.0 + 2.0*eta - mu)/etam1sq;
        beta = (mu - 3.0*eta)/(2.0*etam1sq);
        //C
        //C   CALCULATE THE STRUCTURE FACTOR
        //C
        kk = q*aa;
        k2 = kk*kk;
        k3 = kk*k2;
        SINCOS(kk,ds,dc);
        //ds = sin(kk);
        //dc = cos(kk);
        aq1 = ((ds - kk*dc)*alpha)/k3;
        aq2 = (beta*(1.0-dc))/k2;
        aq3 = (lam*ds)/(12.0*kk);
        aq = 1.0 + 12.0*eta*(aq1+aq2-aq3);
        //
        bq1 = alpha*(0.5/kk - ds/k2 + (1.0 - dc)/k3);
        bq2 = beta*(1.0/kk - ds/k2);
        bq3 = (lam/12.0)*((1.0 - dc)/kk);
        bq = 12.0*eta*(bq1+bq2-bq3);
        //
        sq = 1.0/(aq*aq +bq*bq);

        return(sq);
"""
Iqxy = """
    // never called since no orientation or magnetic parameters.
    return -1.0;
    """

# ER defaults to 0.0
# VR defaults to 1.0

oldname = 'StickyHSStructure'
oldpars = dict()
demo = dict(effect_radius=200, volfraction=0.2, perturb=0.05,
            stickiness=0.2, effect_radius_pd=0.1, effect_radius_pd_n=40)