[gmx-users] Deviations in free energies with slow growth (single and 3-step process)

Maik Goette mgoette at mpi-bpc.mpg.de
Mon Feb 4 12:33:12 CET 2008

Hi all

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 
"single-step switching".
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...

Any ideas?


Maik Goette, Dipl. Biol.
Max Planck Institute for Biophysical Chemistry
Theoretical & computational biophysics department
Am Fassberg 11
37077 Goettingen
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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