Made oxDNA configuration writer use arbdmodel.debye_length if salt_concentration not provided

mrdna/model/arbdmodel.py

@@ -1255,7 +1255,7 @@ E = 0 0 0
                            log_file,
                            num_steps = 1e6,
                            interaction_type = 'DNA2',
-                           salt_concentration = 0.5,
+                           salt_concentration = None,
                            print_conf_interval = None,
                            print_energy_every = None,
                            timestep = 0.003,
@@ -1294,7 +1294,14 @@ E = 0 0 0
         if max_backbone_force is not None:
             max_backbone_force = 'max_backbone_force = {}'.format(max_backbone_force)
 
+
         if interaction_type in ('DNA2',):
+            if salt_concentration is None:
+                try:
+                    ## units "80 epsilon0 295 k K / (2 (AA)**2 e**2/particle)" mM
+                    salt_concentration = 9331.3126/self.debye_length**2
+                except:
+                    salt_concentration = 0.5
             salt_concentration = 'salt_concentration = {}'.format(salt_concentration)
         else:
             salt_concentration = ''