diff --git a/painter.py b/painter.py
index 9974cf9..8db1e89 100644
--- a/painter.py
+++ b/painter.py
@@ -1,4 +1,5 @@
 import sys
+import math
 from kivy.uix.widget import Widget
 from kivy.core.window import Window
 
@@ -6,6 +7,8 @@ from point import Point
 from polyomino import Square
 from polyomino import Square_element
 
+from tools import remove_fromlist
+
 # painter class
 class Painter(Widget):
 
@@ -19,6 +22,8 @@ class Painter(Widget):
 
         # list of selected particles
         self.selected=[]
+        # complement
+        self.unselected=self.particles
 
         # relative position of mouse when moving
         self.offset=Point(0,0)
@@ -85,6 +90,7 @@ class Painter(Widget):
                     for sel in self.selected:
                         sel.selected=False
                     self.selected=[]
+                    self.unselected=self.particles
 
                     self.draw()
 
@@ -99,11 +105,20 @@ class Painter(Widget):
 
                 # no modifiers
                 if self.modifiers==[]:
-                    if self.undermouse==None or not self.undermouse in self.selected:
+                    if self.undermouse==None:
                         # unselect all particles
                         for sel in self.selected:
                             sel.selected=False
                         self.selected=[]
+                        self.unselected=self.particles
+                    # select undermouse
+                    elif not self.undermouse in self.selected:
+                        for sel in self.selected:
+                            sel.selected=False
+                        self.selected=[self.undermouse]
+                        self.unselected=self.particles.copy()
+                        self.unselected=remove_fromlist(self.unselected,self.undermouse)
+                        self.undermouse.selected=True
 
 
                 # shift-click
@@ -112,11 +127,14 @@ class Painter(Widget):
                         if self.undermouse not in self.selected:
                             self.selected.append(self.undermouse)
                             self.undermouse.selected=True
+                            # remove from unselected
+                            self.unselected=remove_fromlist(self.unselected,self.undermouse)
                         else:
                             # remove
-                            self.selected[self.selected.index(self.undermouse)]=self.selected[len(self.selected)-1]
-                            self.selected=self.selected[:len(self.selected)-1]
+                            self.selected=remove_fromlist(self.selected,self.undermouse)
                             self.undermouse.selected=False
+                            # add to unselected
+                            self.unselected.append(self.undermouse)
 
                 self.draw()
 
@@ -127,42 +145,14 @@ class Painter(Widget):
         if self.collide_point(*touch.pos):
             # only move on left click
             if touch.button=="left" and self.modifiers==[] and self.undermouse!=None:
-                # change in position
-                delta=self.check_move(Point(touch.x/Square_element.size,touch.y/Square_element.size)-(self.offset+self.undermouse.squares[0].pos),self.undermouse)
-                self.undermouse.move(delta)
-                # multiple particles
-                # TODO: group moves
-                #else:
-                #    self.group_move(touch)
+                # attempted move determined by the relative position to the relative position of click within self.undermouse
+                delta=self.adjust_move(Point(touch.x/Square_element.size,touch.y/Square_element.size)-(self.offset+self.undermouse.squares[0].pos))
+                for particle in self.selected:
+                    particle.move(delta)
 
                 # redraw
                 self.draw()
 
-    ## move all selected particles
-    #def group_move(self,touch):
-    #    # save position of undermouse (to use it after it has been reset)
-    #    relative_position=self.undermouse.pos
-
-    #    # determine order in which to move
-    #    # direction of motion
-    #    direction=Point(touch.x/Square_element.size,touch.y/Square_element.size)-self.undermouse.pos
-    #    # sort according to scalar product with direction
-    #    self.selected.sort(key=(lambda particle: direction.dot(particle.pos-self.undermouse.pos)),reverse=True)
-
-    #    # move
-    #    for particle in self.selected:
-    #        particle.pos=self.check_move(Point(touch.x/Square_element.size-relative_position.x+particle.pos.x,touch.y/Square_element.size-relative_position.y+particle.pos.y),particle)
-    #    
-    #    ## new position of undermouse
-    #    #self.undermouse.pos=self.check_move(Point(touch.x/Square_element.size,touch.y/Square_element.size),self.undermouse)
-    #    ## move other particles by the same amount as undermouse
-    #    #for particle in self.selected:
-    #    #    if particle!=self.undermouse:
-    #    #        particle.pos=self.check_move(Point(self.undermouse.pos.x-relative_position.x+particle.pos.x,self.undermouse.pos.y-relative_position.y+particle.pos.y),particle)
-    #    #
-    #    #for particle in self.selected:
-    #    #    particle.pos=self.check_move(Point(touch.x/Square_element.size-relative_position.x+particle.pos.x,touch.y/Square_element.size-relative_position.y+particle.pos.y),particle)
-
 
     # find the particle at position pos
     def find_particle(self,pos):
@@ -172,7 +162,7 @@ class Painter(Widget):
         # none found
         return None
 
-    # check whether a position intersects with any of the particles
+    # check whether a candidate particle intersects with any of the particles
     def check_interaction_any(self,candidate,offset):
         for particle in self.particles:
             # do not check interaction if candidate=particle
@@ -180,32 +170,77 @@ class Painter(Widget):
                 return True
         return False
 
-    # check that a particle can move by delta, and return the closest allowed relative motion
-    def check_move(self,delta,particle):
+    # check whether shifting a list of particles by offset makes them interact with all particles
+    def check_interaction_list(self,array,offset):
+        for candidate in array:
+            if self.check_interaction_any(candidate,offset):
+                return True
+        return False
+
+    # check whether shifting a list of particles by offset makes them interact with the unselected particles
+    def check_interaction_unselected_list(self,array,offset):
+        for candidate in array:
+            for particle in self.unselected:
+                if particle.check_interaction(candidate,offset):
+                    return True
+        return False
+
+    # check whether a candidate particle element with any of the unselected particles
+    def check_interaction_unselected_element(self,element,offset):
+        for particle in self.unselected:
+            for square in particle.squares:
+                if square.check_interaction(element.pos+offset):
+                    return True
+        return False
+
+
+    # try to move all selected particles by delta, adjust if needed to avoid overlap with unselected particles
+    # we only track whether these elements collide with unselected particles, not with each other
+    def adjust_move(self,delta):
+        # actual_delta is the smallest (componentwise) of all the computed delta's
+        actual_delta=Point(math.inf,math.inf)
+        for particle in self.selected:
+            for element in particle.squares:
+                # compute adjustment move due to unselected obstacles
+                adjusted_delta=self.adjust_move_element(delta,element)
+                # only keep the smallest delta's (in absolute value)
+                if abs(adjusted_delta.x)<abs(actual_delta.x):
+                    actual_delta.x=adjusted_delta.x
+                if abs(adjusted_delta.y)<abs(actual_delta.y):
+                    actual_delta.y=adjusted_delta.y
+        # try to move by actual_delta
+        if not self.check_interaction_unselected_list(self.selected,actual_delta):
+            return actual_delta
+        else:
+            # cannot move particles at all, try again
+            return self.adjust_move(actual_delta)
+
+
+    # trying to move a single element by delta, adjust if needed to avoid overlap with unselected particles
+    def adjust_move_element(self,delta,element):
         # whether newpos is acceptable
         accept_newpos=True
-        for other in self.particles:
-            # do not compare a particle to itself
-            if other!=particle:
-                # move would make particle overlap with other
-                if other.check_interaction(particle,delta):
+        for other in self.unselected:
+            for obstacle in other.squares:
+                # move would make element overlap with obstacle
+                if obstacle.check_interaction(element.pos+delta):
                     accept_newpos=False
-                    # check if particle already touches other
-                    if other.check_touch(particle):
-                        # move along other while remaining stuck
-                        # TODO: this assumes other is a square
-                        newdelta=other.squares[0].move_along(delta,particle.squares[0].pos)
+                    # check if particle already touches obstacle
+                    if obstacle.check_touch(element.pos):
+                        # move along obstacle while remaining stuck
+                        newdelta=obstacle.move_along(delta,element.pos)
                     else:
-                        newdelta=other.squares[0].move_on_line_to_stick(particle.squares[0].pos,delta)
-                    if not self.check_interaction_any(particle,newdelta):
+                        newdelta=obstacle.move_on_line_to_stick(element.pos,delta)
+                    if not self.check_interaction_unselected_element(element,newdelta):
                         return newdelta
         if accept_newpos:
             return delta
         else:
             # cannot move particle at all, try again
-            return self.check_move(newdelta,particle)
+            return self.adjust_move_element(newdelta,element)
 
 
+    # TODO adapt
     # write configuration to file
     def write(self,file):
         ff=open(file,"w")
@@ -213,6 +248,7 @@ class Painter(Widget):
             ff.write("{:05.2f},{:05.2f};{:3.1f},{:3.1f},{:3.1f}\n".format(particle.pos.x,particle.pos.y,particle.color[0],particle.color[1],particle.color[2]))
         ff.close()
 
+    # TODO adapt
     # read configuration from file
     def read(self,file):
         self.reset()
diff --git a/polyomino.py b/polyomino.py
index d401759..88996ae 100644
--- a/polyomino.py
+++ b/polyomino.py
@@ -63,15 +63,6 @@ class Polyomino():
                     return True
         return False
 
-    # check whether self touches candidate
-    def check_touch(self,candidate):
-        for square1 in self.squares:
-            for square2 in candidate.squares:
-                if square1.check_touch(square2.pos):
-                    return True
-        return False
-        
-
 # square
 class Square(Polyomino):
     def __init__(self,x,y,**kwargs):
diff --git a/tools.py b/tools.py
index 651da3d..dc661fa 100644
--- a/tools.py
+++ b/tools.py
@@ -13,3 +13,10 @@ def isint_nonzero(x):
 # check that a number is in an interval
 def in_interval(x,a,b):
     return x>=a and x<=b
+
+# remove x from list a
+def remove_fromlist(a,x):
+    if x in a:
+        a[a.index(x)]=a[len(a)-1]
+        a=a[:len(a)-1]
+    return a