diff --git a/python/openEMS/_nf2ff.pxd b/python/openEMS/_nf2ff.pxd
index 7e1b764..859f062 100644
--- a/python/openEMS/_nf2ff.pxd
+++ b/python/openEMS/_nf2ff.pxd
@@ -24,9 +24,9 @@ cimport cython.numeric
 
 cdef extern from "openEMS/nf2ff.h":
     cdef cppclass cpp_nf2ff "nf2ff":
-        cpp_nf2ff(vector[float] freq, vector[float] theta, vector[float] phi, vector[float] center, unsigned int numThreads) except +
+        cpp_nf2ff(vector[float] freq, vector[float] theta, vector[float] phi, vector[float] center, unsigned int numThreads) nogil except +
 
-        bool AnalyseFile(string E_Field_file, string H_Field_file)
+        bool AnalyseFile(string E_Field_file, string H_Field_file) nogil
 
         void SetRadius(float radius)
         void SetPermittivity(vector[float] permittivity);
@@ -41,7 +41,7 @@ cdef extern from "openEMS/nf2ff.h":
         complex[double]** GetEPhi(size_t f_idx)
         double** GetRadPower(size_t f_idx)
 
-        bool Write2HDF5(string filename)
+        bool Write2HDF5(string filename) nogil
 
         void SetVerboseLevel(int level)
 
diff --git a/python/openEMS/_nf2ff.pyx b/python/openEMS/_nf2ff.pyx
index 8016694..14408b9 100644
--- a/python/openEMS/_nf2ff.pyx
+++ b/python/openEMS/_nf2ff.pyx
@@ -40,7 +40,11 @@ cdef class _nf2ff:
     def AnalyseFile(self, e_file, h_file):
         assert os.path.exists(e_file)
         assert os.path.exists(h_file)
-        return self.thisptr.AnalyseFile(e_file.encode('UTF-8'), h_file.encode('UTF-8'))
+        cdef string e_fn = e_file.encode('UTF-8')
+        cdef string h_fn = h_file.encode('UTF-8')
+        with nogil:
+            ok = self.thisptr.AnalyseFile(e_fn, h_fn)
+        return ok
 
     def SetMirror(self, mirr_type, ny, pos):
         if mirr_type<=0:
diff --git a/python/openEMS/openEMS.pxd b/python/openEMS/openEMS.pxd
index a307152..d9ef0e9 100644
--- a/python/openEMS/openEMS.pxd
+++ b/python/openEMS/openEMS.pxd
@@ -23,7 +23,7 @@ from CSXCAD.CSXCAD cimport _ContinuousStructure, ContinuousStructure
 
 cdef extern from "openEMS/openems.h":
     cdef cppclass _openEMS "openEMS":
-        _openEMS() except +
+        _openEMS() nogil except +
         void SetNumberOfTimeSteps(unsigned int val)
         void SetCSX(_ContinuousStructure* csx)
 
@@ -49,13 +49,15 @@ cdef extern from "openEMS/openems.h":
 
         void SetGaussExcite(double f0, double fc)
 
-        void SetVerboseLevel(int level)
-        void DebugPEC()
-        void DebugMaterial()
-        void DebugCSX()
+        void SetAbort(bool val)
 
-        int SetupFDTD()
-        void RunFDTD()
+        void SetVerboseLevel(int level)
+        void DebugPEC()      nogil
+        void DebugMaterial() nogil
+        void DebugCSX()      nogil
+
+        int SetupFDTD() nogil
+        void RunFDTD()  nogil
 
         @staticmethod
         void WelcomeScreen()
diff --git a/python/openEMS/openEMS.pyx b/python/openEMS/openEMS.pyx
index 4d2aff3..44bc493 100644
--- a/python/openEMS/openEMS.pyx
+++ b/python/openEMS/openEMS.pyx
@@ -432,15 +432,21 @@ cdef class openEMS:
         if verbose is not None:
             self.thisptr.SetVerboseLevel(verbose)
         if debug_pec:
-            self.thisptr.DebugPEC()
+            with nogil:
+                self.thisptr.DebugPEC()
         if 'numThreads' in kw:
             self.thisptr.SetNumberOfThreads(int(kw['numThreads']))
         assert os.getcwd() == sim_path
         _openEMS.WelcomeScreen()
         cdef int EC
-        EC = self.thisptr.SetupFDTD()
+        with nogil:
+            EC = self.thisptr.SetupFDTD()
         if EC!=0:
             print('Run: Setup failed, error code: {}'.format(EC))
         if setup_only or EC!=0:
             return EC
-        self.thisptr.RunFDTD()
+        with nogil:
+            self.thisptr.RunFDTD()
+
+    def SetAbort(self, val):
+        self.thisptr.SetAbort(val)