adding peak_voigt model

sasmodels/models/peak_voigt.py

@@ -0,0 +1,149 @@
+r"""
+This model describes a pseudo-Voigt shaped peak on a flat background.
+
+Definition
+----------
+
+This pseudo-Voigt peak function is a weighted linear summation of
+Lorentzian (L) and Gaussian (G) peak shapes. 
+It is a popular function for modelling peak shape.
+It can be tailored to any specific peak shape and it can also produce a peak shape with asymmetry. 
+
+The scattering intensity $I(q)$ is calculated as
+
+.. math::
+
+    I(q) = scale \cdot \left[ w_f \cdot I(q)_L + (1 - w_f) \cdot I(q)_G \right] + background
+
+where $w_f$ is a weighting factor and
+
+.. math::
+
+    I(q)_L = \frac{1}{1 + \left( \frac{q - q_0}{HWHM} \right)^2}
+
+    I(q)_G = \exp\left[ -\frac{1}{2} (q - q_0)^2 / \sigma^2 \right]
+
+The peak is taken to be centered at $q_0$ with a HWHM (half-width
+half-maximum) of $1.17741\,\sigma$, where $\sigma$ is the standard deviation
+of the Gaussian. In other words, the widths of the Lorentzian and the
+Gaussian have been coupled for convenience of parameterisation:
+
+.. math::
+
+    \sigma = HWHM / \sqrt{2 \ln 2} = HWHM / 1.17741
+
+When $w_f = 1$ a Lorentzian peak is returned, and when $w_f = 0$ a
+Gaussian peak is returned.
+
+For 2D data the 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}
+
+
+Validation
+----------
+
+The pseudo-Voigt peak reduces exactly to a pure Lorentzian for $w_f = 1$
+and to a pure Gaussian for $w_f = 0$; both limits were checked against their
+analytic values (see tests section at the end).
+The full pseudo-Voigt shape has also been compared, for identical
+parameters, against a slightly different SasView implementation (https://marketplace.sasview.org/models/127/) 
+of the same function and gives the same result. 
+
+
+References
+----------
+
+1. L A Feigin, D I Svergun, G W Taylor
+   Structure Analysis by Small-Angle X-ray and Neutron Scattering
+   Springer (1987)
+
+2. Aaron L. Stancik, Eric B. Brauns
+   A simple asymmetric lineshape for fitting infrared absorption spectra
+   Vibrational Spectroscopy 47 (2008) 66-69
+
+
+Authorship and Verification
+----------------------------
+
+* **Author:**  Steve King **Date:** 24 June 2020
+
+* **Authors:** Marianne Imperor-Clerc (marianne.imperor@cnrs.fr)
+               Anirban Mandal (mandalanirban2023@gmail.com)
+
+* **Last Modified by:** Anirban Mandal **Date:** 06 July 2026
+
+* **Last Reviewed by:** Steve King **Date:**
+
+"""
+
+import numpy as np
+from numpy import errstate, inf
+
+name = "peak_voigt"
+title = "Single pseudo-Voigt peak"
+description = """\
+          I(q) = scale*peak + background
+"""
+
+category = "shape-independent"
+
+parameters = [["w_f", "", 0.8, [0, 1], "", "lorentzian/gaussian weighting factor"],
+              ["peak_pos", "1/Ang", 0.05, [0, inf], "", "Position of the peak"],
+              ["peak_hwhm", "1/Ang", 0.01, [0, 1], "", "HWHM of the peak"]]
+
+
+def Ipeak(q, wf, q0, hwhm):
+    """
+    When $w_f$ = 1 a Lorentzian peak is returned, and when $w_f$ = 0 a
+    Gaussian peak is returned.
+
+    The peak is taken to be centered at $q_0$ with a HWHM (half-width
+    half-maximum) for the Lorentzian and sigma = HWHM / 1.17741 for the
+    Gaussian, where sigma is the standard deviation of the Gaussian. In
+    other words, the widths of the Lorentzian and the Gaussian have been
+    coupled for convenience of parameterisation.
+    """
+    cste = np.sqrt(2 * np.log(2))
+    # cste = 1.17741
+    sigma = hwhm / cste
+    intensity = (wf * (1 / (1 + ((q - q0)**2.0 / hwhm**2.0)))) + \
+                ((1.0 - wf) * np.exp((-0.5 * (q - q0)**2.0) / (sigma**2.0)))
+    return intensity
+
+
+def Iq(q, w_f, peak_pos, peak_hwhm):
+    """
+    w_f: weighting coefficient in the pseudo-Voigt peak function;
+    w_f = 1 for a Lorentzian and w_f = 0 for a Gaussian peak.
+    peak_pos: position of the peak
+    peak_hwhm: HWHM of the peak
+    """
+
+    with errstate(divide='ignore'):
+        L = Ipeak(q, w_f, peak_pos, peak_hwhm)
+
+        return L
+
+Iq.vectorized = True  # Iq accepts an array of q values
+
+tests = [
+    # pure Lorentzian (w_f = 1): peak centre, half-width, and 2 x HWHM
+    [{"scale": 1.0, "background": 0.0, "w_f": 1.0,
+      "peak_pos": 0.05, "peak_hwhm": 0.01}, 0.05, 1.0],
+    [{"scale": 1.0, "background": 0.0, "w_f": 1.0,
+      "peak_pos": 0.05, "peak_hwhm": 0.01}, 0.06, 0.5],
+    [{"scale": 1.0, "background": 0.0, "w_f": 1.0,
+      "peak_pos": 0.05, "peak_hwhm": 0.01}, 0.07, 0.2],
+    # pure Gaussian (w_f = 0): half-width is 0.5 by definition, 2 x HWHM = 1/16
+    [{"scale": 1.0, "background": 0.0, "w_f": 0.0,
+      "peak_pos": 0.05, "peak_hwhm": 0.01}, 0.06, 0.5],
+    [{"scale": 1.0, "background": 0.0, "w_f": 0.0,
+      "peak_pos": 0.05, "peak_hwhm": 0.01}, 0.07, 0.0625],
+    # mixed pseudo-Voigt (w_f = 0.8) away from the centre
+    [{"scale": 1.0, "background": 0.0, "w_f": 0.8,
+      "peak_pos": 0.05, "peak_hwhm": 0.01}, 0.07, 0.1725],
+]