diff --git a/src/QMCDrivers/WFOpt/QMCCostFunctionBase.h b/src/QMCDrivers/WFOpt/QMCCostFunctionBase.h
index 05e8111423..a7045b2405 100644
--- a/src/QMCDrivers/WFOpt/QMCCostFunctionBase.h
+++ b/src/QMCDrivers/WFOpt/QMCCostFunctionBase.h
@@ -176,6 +176,13 @@ class QMCCostFunctionBase : public CostFunctionBase<QMCTraits::RealType>, public
 
   inline const opt_variables_type& getOptVariables() const { return OptVariables; }
 
+  /// return variance after checkConfigurations
+  inline Return_rt getVariance() const
+  {
+    return SumValue[SUM_ESQ_WGT] / SumValue[SUM_WGT] -
+        (SumValue[SUM_E_WGT] / SumValue[SUM_WGT]) * (SumValue[SUM_E_WGT] / SumValue[SUM_WGT]);
+  }
+
 protected:
   ///walker ensemble
   MCWalkerConfiguration& W;
diff --git a/src/QMCDrivers/WFOpt/QMCLinearOptimize.cpp b/src/QMCDrivers/WFOpt/QMCLinearOptimize.cpp
index 7126d19454..c27a05c467 100644
--- a/src/QMCDrivers/WFOpt/QMCLinearOptimize.cpp
+++ b/src/QMCDrivers/WFOpt/QMCLinearOptimize.cpp
@@ -79,28 +79,35 @@ void QMCLinearOptimize::addConfiguration(const std::string& a)
 
 void QMCLinearOptimize::start()
 {
-  //close files automatically generated by QMCDriver
-  //     branchEngine->finalize();
-  //generate samples
-  generate_samples_timer_.start();
-  generateSamples();
-  generate_samples_timer_.stop();
-  //store active number of walkers
-  NumOfVMCWalkers = W.getActiveWalkers();
+  {
+    //generate samples
+    ScopedTimer local(&generate_samples_timer_);
+    generateSamples();
+    //store active number of walkers
+    NumOfVMCWalkers = W.getActiveWalkers();
+  }
+
   app_log() << "<opt stage=\"setup\">" << std::endl;
   app_log() << "  <log>" << std::endl;
   //reset the rootname
   optTarget->setRootName(RootName);
   optTarget->setWaveFunctionNode(wfNode);
   app_log() << "   Reading configurations from h5FileRoot " << h5FileRoot << std::endl;
-  //get configuration from the previous run
-  Timer t1;
-  initialize_timer_.start();
-  optTarget->getConfigurations(h5FileRoot);
-  optTarget->setRng(vmcEngine->getRng());
-  optTarget->checkConfigurations();
-  initialize_timer_.stop();
-  app_log() << "  Execution time = " << std::setprecision(4) << t1.elapsed() << std::endl;
+  {
+    //get configuration from the previous run
+    ScopedTimer local(&initialize_timer_);
+    Timer t2;
+    optTarget->getConfigurations(h5FileRoot);
+    optTarget->setRng(vmcEngine->getRng());
+    optTarget->checkConfigurations();
+    // check recomputed variance against VMC
+    auto sigma2_vmc   = vmcEngine->getBranchEngine()->vParam[SimpleFixedNodeBranch::SBVP::SIGMA2];
+    auto sigma2_check = optTarget->getVariance();
+    if (sigma2_check > 2.0 * sigma2_vmc || sigma2_check < 0.5 * sigma2_vmc)
+      throw std::runtime_error(
+          "Safeguard failure: checkConfigurations variance out of [0.5, 2.0] * reference! Please report this bug.\n");
+    app_log() << "  Execution time = " << std::setprecision(4) << t2.elapsed() << std::endl;
+  }
   app_log() << "  </log>" << std::endl;
   app_log() << "</opt>" << std::endl;
   app_log() << "<opt stage=\"main\" walkers=\"" << optTarget->getNumSamples() << "\">" << std::endl;
@@ -115,13 +122,14 @@ void QMCLinearOptimize::engine_start(cqmc::engine::LMYEngine<ValueType>* EngineO
 {
   app_log() << "entering engine_start function" << std::endl;
 
-  // generate samples
-  generate_samples_timer_.start();
-  generateSamples();
-  generate_samples_timer_.stop();
+  {
+    //generate samples
+    ScopedTimer local(&generate_samples_timer_);
+    generateSamples();
+    //store active number of walkers
+    NumOfVMCWalkers = W.getActiveWalkers();
+  }
 
-  // store active number of walkers
-  NumOfVMCWalkers = W.getActiveWalkers();
   app_log() << "<opt stage=\"setup\">" << std::endl;
   app_log() << "  <log>" << std::endl;
 
@@ -129,16 +137,16 @@ void QMCLinearOptimize::engine_start(cqmc::engine::LMYEngine<ValueType>* EngineO
   optTarget->setRootName(RootName);
   optTarget->setWaveFunctionNode(wfNode);
   app_log() << "     Reading configurations from h5FileRoot " << h5FileRoot << std::endl;
-
-  // get configuration from the previous run
-  Timer t1;
-  initialize_timer_.start();
-  optTarget->getConfigurations(h5FileRoot);
-  optTarget->setRng(vmcEngine->getRng());
-  optTarget->engine_checkConfigurations(EngineObj, descentEngineObj,
-                                        MinMethod); // computes derivative ratios and pass into engine
-  initialize_timer_.stop();
-  app_log() << "  Execution time = " << std::setprecision(4) << t1.elapsed() << std::endl;
+  {
+    // get configuration from the previous run
+    ScopedTimer local(&initialize_timer_);
+    Timer t2;
+    optTarget->getConfigurations(h5FileRoot);
+    optTarget->setRng(vmcEngine->getRng());
+    optTarget->engine_checkConfigurations(EngineObj, descentEngineObj,
+                                          MinMethod); // computes derivative ratios and pass into engine
+    app_log() << "  Execution time = " << std::setprecision(4) << t2.elapsed() << std::endl;
+  }
   app_log() << "  </log>" << std::endl;
   app_log() << "</opt>" << std::endl;
   app_log() << "<opt stage=\"main\" walkers=\"" << optTarget->getNumSamples() << "\">" << std::endl;
diff --git a/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp b/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp
index 25fccc71af..cacf399afd 100644
--- a/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp
+++ b/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp
@@ -643,6 +643,9 @@ void DiracDeterminant<DU_TYPE>::recompute(ParticleSet& P)
   {
     invertPsiM(psiM_temp, psiM);
   }
+
+  // invRow becomes invalid after updating the inverse matrix
+  invRow_id = -1;
 }
 
 template<typename DU_TYPE>