[gmx-users] Deviations in free energies with slow growth (single and 3-step process)
mgoette at mpi-bpc.mpg.de
Mon Feb 4 12:33:12 CET 2008
I ran into a problem while testing some free energy methods.
I simulated a switching process (slow growth TI) over 10ns of ethane to
methanol with hardcore slow-growth in water (365 TIP4P waters,PME) and
in vacuum. The thermodynamic cycle of this calculation yields a DeltaG,
which is in perfect agreement with the experiment and other calculations.
Now, when splitting this simulation into 3 steps (3x 3.2ns), where I
switch the charges to zero in the first step (hardcore), morph the LJ
and bonded in the second (softcore or hardcore) and switch the charges
back on from zero in the third step (hardcore), all in solvent, the sum
of the single contributions does NOT yield the same number as the
In vacuum, the work values I get from the single step process and from
adding up the values from the 3-step process perfectly match.
The topologies for all systems are the same (except +/- water) for the
1-step and the 3-step simulations.
Now I'm a bit puzzled and can't get an idea, where this difference in
the solvated system may come from. I exclude a problem due to softcore,
cause I also simulated the 3-step process all hardcore...
Maik Goette, Dipl. Biol.
Max Planck Institute for Biophysical Chemistry
Theoretical & computational biophysics department
Am Fassberg 11
Tel. : ++49 551 201 2310
Fax : ++49 551 201 2302
Email : mgoette[at]mpi-bpc.mpg.de
WWW : http://www.mpibpc.gwdg.de/groups/grubmueller/
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