diff --git a/configure b/configure
index 62f58c48..9aec9834 100755
--- a/configure
+++ b/configure
@@ -3,7 +3,7 @@
 # Quantum Package configuration script
 #
 
-export QP_ROOT="$( cd "$(dirname "$0")" ; pwd -P )"
+export QP_ROOT="$( cd "$(dirname "$0")" ; pwd --physical )"
 echo "QP_ROOT="$QP_ROOT
 
 
diff --git a/scripts/qp_bitmasks.py b/scripts/qp_bitmasks.py
old mode 100644
new mode 100755
index ff97edd3..223c32ba
--- a/scripts/qp_bitmasks.py
+++ b/scripts/qp_bitmasks.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python2
+#!/usr/bin/env python2
 
 BIT_KIND_SIZE=64
 
@@ -24,7 +24,7 @@ def int_to_string(s):
     """
     assert type(s) in (int, long)
     assert s>=0
-    return str(s) if s in (0,1) else int_to_string(s>>1) + str(s&1) 
+    return '{s:0b}'.format(s=s)
 
 
 def string_to_bitmask(s,bit_kind_size=BIT_KIND_SIZE):
@@ -63,15 +63,9 @@ def int_to_bitmask(s,bit_kind_size=BIT_KIND_SIZE):
     >>> 
     """
     assert type(s) in (int, long)
-    if s>=0:
-      s = int_to_string(s)
-      result = string_to_bitmask( s, bit_kind_size )
-    else:
-      s = int_to_string(-s-1)
-      result = string_to_bitmask( s, bit_kind_size )
-      result = [ x.replace('1','.').replace('0','1').replace('.','0') for x in result ]
-    return result
-
+    if s < 0:
+        s = s + (1 << bit_kind_size)
+    return ['{s:0{width}b}'.format(s=s,width=bit_kind_size)]
 
 
 class BitMask(object):
@@ -102,9 +96,9 @@ class BitMask(object):
     self.bit_kind_size = bit_kind_size
     self._data_int = l
 
-  def get_N_int(self):
+  @property
+  def N_int(self):
     return len(self._data_int)
-  N_int = property(fget=get_N_int)
 
   def __getitem__(self,i):
     return self._data_int[i]
@@ -126,7 +120,12 @@ class BitMask(object):
       result += int_to_bitmask(i,bit_kind_size=self.bit_kind_size)
     return str(result)
 
-
+def excitation_degree(l_a,l_b):
+  '''
+  excitation_degree([895],[959])
+  >> 1
+  '''
+  return sum(bin(a ^ b).count("1") for a,b in zip(l_a,l_b) ) // 2
 
 if __name__ == '__main__':
   import doctest