diff --git a/simulation_TREPR.m b/simulation_TREPR.m
index 1bf53f8..24dc2b8 100644
--- a/simulation_TREPR.m
+++ b/simulation_TREPR.m
@@ -1,8 +1,19 @@
-function [params] = simulation_TREPR(Exp,Sys0,params)
+function [params] = simulation_TREPR(params)
 %SIMULATION_TREPR Summary of this function goes here
 %
 %   Detailed explanation goes here
 
+%% setup
+%create struct for passing data between callbacks
+eventdata.manSys = params.Manual_Sys;
+eventdata.manExp.mwFreq = params.mwFreq; %GHz
+eventdata.manExp.nPoints= length(params.Field_Vector);
+eventdata.manExp.CenterSweep = 0.1*[params.Field_Center params.Field_Sweep]; % mT (converted from Gauss)
+eventdata.manExp.Harmonic = 0; % zeroth harmonic
+eventdata.manExp.Temperature = params.Triplett_pop;
+eventdata.simSys = params.Manual_Sys; %initialize simulation parameters
+eventdata.simExp = eventdata.manExp;
+
 %% creating UI
 fig = figure;
 fig.Name = 'Simulation';
@@ -19,7 +30,7 @@ ax = axes(plotpanel);
 ax.XLabel.String = 'magnetic field';
 ax.YLabel.String = 'Intensity';
 %plot manual simulation
-[bfield,spec] = pepper(Sys0,Exp);
+[bfield,spec] = pepper(eventdata.manSys,eventdata.manExp);
 spec_norm = spec/max(spec); % normalize the simulation
 plot(ax,0.1*params.Field_Vector,params.max_mean,'r',...
      bfield,spec_norm,'b','LineWidth',1.2)
@@ -286,7 +297,7 @@ uicontrol(InpTx); %passes focus to first input
 uiwait(fig)
 
 %% Callback functions
-    function BestSys = SimStartButtonPushed(src,event)
+    function SimStartButtonPushed(src,event)
         %get Vary and Opt from Input
         varT1 = str2double(get(InpTx,'String'));
         varT2 = str2double(get(InpTy,'String'));
@@ -301,19 +312,31 @@ uiwait(fig)
         Vary.g = str2double(get(InpG,'String'));
         FitOpt.Method = get(InpMeth,'String');
         FitOpt.Scaling = get(InpScal,'String');
+        %get previous simulation parameters (manual or saved)
+        Sys0 = eventdata.simSys;
+        Exp0 = eventdata.simExp;
         %perform fit
         Sys0.Temperature = [0 0 0];
-        [BestSys,BestSpc] = esfit('thyme',params.max_mean,Sys0,Vary,Exp,[],FitOpt);
+        [BestSys,BestSpc] = esfit('thyme',params.max_mean,Sys0,Vary,Exp0,[],FitOpt);
         plot(ax,0.1*params.Field_Vector,params.max_mean,'r',...
                 0.1*params.Field_Vector,BestSpc,'b','LineWidth',1.2)
         xlabel 'Magnetic Field / mT'
         ylabel 'EPR signal / A. U.'
         legend('experimental','simulation')
         axis tight
+        %normalize Triplett population
+        Tsum = sum(BestSys.Temperature);
+        BestSys.Temperature = BestSys.Temperature ./ Tsum;
+        %write best simulation parameters to eventdata
+        eventdata.BestSys = BestSys;
     end
     
     function SimSaveButtonPushed(src,event)
-%         Sys = BestSys;
+        %set current parameters as new start values
+        eventdata.simSys = eventdata.BestSys;
+        eventdata.simExp.Temperature = eventdata.BestSys.Temperature;
+        %save fitted parameters to struct
+        params.Fitted_Simulation = eventdata.BestSys;
     end
     
     function SimDoneButtonPushed(src,event)