From fecb8914669d3bb6495437b4cbef0a423f34d4bf Mon Sep 17 00:00:00 2001
From: Applin <robert.applin@stfc.ac.uk>
Date: Thu, 11 Jul 2024 16:52:21 +0100
Subject: [PATCH 1/2] Use original sample to normalise randomized sample

---
 .../inc/MantidAlgorithms/CalculateIqt.h       |  6 +++--
 Framework/Algorithms/src/CalculateIqt.cpp     | 23 +++++++++++--------
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/Framework/Algorithms/inc/MantidAlgorithms/CalculateIqt.h b/Framework/Algorithms/inc/MantidAlgorithms/CalculateIqt.h
index 0d6c9590e36f..6f56e451f766 100644
--- a/Framework/Algorithms/inc/MantidAlgorithms/CalculateIqt.h
+++ b/Framework/Algorithms/inc/MantidAlgorithms/CalculateIqt.h
@@ -43,8 +43,7 @@ class MANTID_ALGORITHMS_DLL CalculateIqt final : public API::Algorithm {
   API::MatrixWorkspace_sptr replaceSpecialValues(const API::MatrixWorkspace_sptr &workspace);
 
   API::MatrixWorkspace_sptr removeInvalidData(API::MatrixWorkspace_sptr workspace);
-  API::MatrixWorkspace_sptr normalizedFourierTransform(API::MatrixWorkspace_sptr workspace,
-                                                       const std::string &rebinParams, const bool enforceNormalization);
+  API::MatrixWorkspace_sptr fourierTransform(API::MatrixWorkspace_sptr workspace, const std::string &rebinParams);
   API::MatrixWorkspace_sptr calculateIqt(API::MatrixWorkspace_sptr workspace,
                                          const API::MatrixWorkspace_sptr &resolutionWorkspace,
                                          const std::string &rebinParams, const bool enforceNormalization);
@@ -54,6 +53,9 @@ class MANTID_ALGORITHMS_DLL CalculateIqt final : public API::Algorithm {
   API::MatrixWorkspace_sptr
   setErrorsToStandardDeviation(const std::vector<API::MatrixWorkspace_sptr> &simulatedWorkspaces);
   API::MatrixWorkspace_sptr setErrorsToZero(const std::vector<API::MatrixWorkspace_sptr> &simulatedWorkspaces);
+
+  API::MatrixWorkspace_sptr m_sampleIntegral;
+  API::MatrixWorkspace_sptr m_resolutionIntegral;
 };
 
 } // namespace Algorithms
diff --git a/Framework/Algorithms/src/CalculateIqt.cpp b/Framework/Algorithms/src/CalculateIqt.cpp
index d31050e0b67b..02f3723cb022 100644
--- a/Framework/Algorithms/src/CalculateIqt.cpp
+++ b/Framework/Algorithms/src/CalculateIqt.cpp
@@ -138,7 +138,14 @@ void CalculateIqt::exec() {
   bool enforceNormalization = getProperty("EnforceNormalization");
   const int nIterations = getProperty("NumberOfIterations");
   const int seed = getProperty("SeedValue");
-  resolution = normalizedFourierTransform(resolution, rebinParams, enforceNormalization);
+
+  m_sampleIntegral = integration(rebin(sampleWorkspace, rebinParams));
+  m_resolutionIntegral = integration(rebin(resolution, rebinParams));
+
+  resolution = fourierTransform(resolution, rebinParams);
+  if (enforceNormalization) {
+    resolution = divide(resolution, m_resolutionIntegral);
+  }
 
   auto outputWorkspace = monteCarloErrorCalculation(sampleWorkspace, resolution, rebinParams, seed, calculateErrors,
                                                     nIterations, enforceNormalization);
@@ -265,24 +272,20 @@ MatrixWorkspace_sptr CalculateIqt::replaceSpecialValues(const MatrixWorkspace_sp
   return specialValuesAlgorithm->getProperty("OutputWorkspace");
 }
 
-MatrixWorkspace_sptr CalculateIqt::normalizedFourierTransform(MatrixWorkspace_sptr workspace,
-                                                              const std::string &rebinParams,
-                                                              const bool enforceNormalization) {
+MatrixWorkspace_sptr CalculateIqt::fourierTransform(MatrixWorkspace_sptr workspace, const std::string &rebinParams) {
   workspace = rebin(workspace, rebinParams);
-  auto workspace_int = integration(workspace);
   workspace = convertToPointData(workspace);
   workspace = extractFFTSpectrum(workspace);
-  if (enforceNormalization) {
-    return divide(workspace, workspace_int);
-  }
-  // return the output workspace of ExtractFFTSpectrum
   return workspace;
 }
 
 MatrixWorkspace_sptr CalculateIqt::calculateIqt(MatrixWorkspace_sptr workspace,
                                                 const MatrixWorkspace_sptr &resolutionWorkspace,
                                                 const std::string &rebinParams, const bool enforceNormalization) {
-  workspace = normalizedFourierTransform(workspace, rebinParams, enforceNormalization);
+  workspace = fourierTransform(workspace, rebinParams);
+  if (enforceNormalization) {
+    workspace = divide(workspace, m_sampleIntegral);
+  }
   return divide(workspace, resolutionWorkspace);
 }
 

From 01996adc81e7e3b91530f33727e011ab54d3f345 Mon Sep 17 00:00:00 2001
From: Applin <robert.applin@stfc.ac.uk>
Date: Mon, 15 Jul 2024 08:28:08 +0100
Subject: [PATCH 2/2] Add release note

---
 docs/source/release/v6.11.0/Inelastic/Bugfixes/37653.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 docs/source/release/v6.11.0/Inelastic/Bugfixes/37653.rst

diff --git a/docs/source/release/v6.11.0/Inelastic/Bugfixes/37653.rst b/docs/source/release/v6.11.0/Inelastic/Bugfixes/37653.rst
new file mode 100644
index 000000000000..93033c0353e0
--- /dev/null
+++ b/docs/source/release/v6.11.0/Inelastic/Bugfixes/37653.rst
@@ -0,0 +1 @@
+- Fixed a bug in the Monte Carlo error calculation on the I(Q, t) tab of the :ref:`Data Processor Interface <interface-inelastic-data-processor>` where the first bin had an error of zero.
\ No newline at end of file