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MsdGauss.py
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MsdGauss.py
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# pylint: disable=no-init,invalid-name
'''
@author Spencer Howells, ISIS
@date December 05, 2013
Copyright © 2007-8 ISIS Rutherford Appleton Laboratory, NScD Oak Ridge National Laboratory & European Spallation Source
This file is part of Mantid.
Mantid is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
Mantid is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
File change history is stored at: <https://github.com/mantidproject/mantid>
Code Documentation is available at: <http://doxygen.mantidproject.org>
'''
from __future__ import (absolute_import, division, print_function)
import math
import numpy as np
from mantid.api import IFunction1D, FunctionFactory
# For a Gaussian distribution the elastic intensity is propotional to exp(-msd*Q^2)
# where the mean square displacement msd = <r^2>.
class MsdGauss(IFunction1D):
def category(self):
return "QuasiElastic"
def init(self):
# Active fitting parameters
self.declareParameter("Height", 1.0, 'Height')
self.declareParameter("MSD", 0.05, 'Mean square displacement')
def function1D(self, xvals):
height = self.getParameterValue("Height")
msd = self.getParameterValue("MSD")
xvals = np.array(xvals)
intensity = height * np.exp(-msd * xvals**2)
return intensity
def functionDeriv1D(self, xvals, jacobian):
height = self.getParameterValue("Height")
msd = self.getParameterValue("MSD")
for i, x in enumerate(xvals):
e = math.exp(-msd * x**2)
jacobian.set(i, 0, e)
jacobian.set(i, 1, -(x**2) * e * height)
i += 1
# Required to have Mantid recognise the new function
FunctionFactory.subscribe(MsdGauss)