[MMTK] Ewald sums.

Adam Hixson chixson@chemdept.chem.ou.edu
Sun, 9 Sep 2001 13:49:55 -0700 (PDT)


I created a box of water using MMTK v2.2b6 using
the following code:

---->
from MMTK import *
from MMTK.ForceFields import Amber94ForceField
from MMTK.Random import randomPointInBox
from MMTK.Solvation import *

size = 30.*Units.Ang

universe = CubicPeriodicUniverse(size,
Amber94ForceField(size / 2.5,None))
universe.addObject(Molecule('water',position=Vector(0,0,0)))
print "...and let there be a universe."
addSolvent(universe,'water',1.*Units.kg/Units.l)
print "Solvent added, and now shrinking..."
save(universe,"preshrunkwater.univ")
shrinkUniverse(universe,temperature=300.*Units.K)
save(universe,"water.univ")
print "Done...and saved."
-----

But when I load this universe and try to evaluate
the energy, it gives a value that is very large.

For example, after loading python:
-----
>>> from MMTK import *
>>> universe = load("water.univ")
>>> universe.energy()
1077872366.1194236
>>> from MMTK.ForceFields import Amber94ForceField
>>>universe.setForceField(Amber94ForceField(15.*Units.Ang,
15.*Units.Ang))
>>> universe.energy()
-33920.758650668249

-----

Doing away with the Ewald sums, and setting a cutoff
gives a more reasonable result.

I am fairly new to MD, and have never used Ewald sums
before, so I may be naively doing somthing bone-
headed.  What might cause this result?

=====
----
Adam Hixson
Department of Chemistry and Biochemistry
The University of Oklahoma     chixson@chemdept.chem.ou.edu
620 Parrington Oval            hixsonc@yahoo.com
Norman, OK 73069

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