diff --git a/mrdna/segmentmodel.py b/mrdna/segmentmodel.py
index 1b9fb8476f4fd811778c077884a54e1bfefa0e08..d4333ed4588c658c720e3c3ea32f640eebfc8023 100644
--- a/mrdna/segmentmodel.py
+++ b/mrdna/segmentmodel.py
@@ -1963,6 +1963,8 @@ class SegmentModel(ArbdModel):
                 t.__dict__["nts"] = bead.num_nt*2 if char in ("D","O") else bead.num_nt
                 # t.name = t.name + "%03d" % (t.nts*10**decimals)
                 t.name = char + "%03d" % (beadtype_count[char])
+                t.mass = t.nts * 150
+                t.diffusivity = 150 if t.nts == 0 else min( 50 / np.sqrt(t.nts/5), 150)
                 beadtype_count[char] += 1
                 if self.DEBUG: print( "{} --> {} ({})".format(num_nt0, bead.num_nt, t.name) )
                 beadtype_s[key] = bead.type_ = t