[gmx-users] Slow-Growth Pressure/System Volume Anomaly

[David L. Bostick]

Hello all,

I am simulating a protein in a hydrated DPPC bilayer.  After building the
system and allowing it to come to equilibration, I used the slow-growth
algorithm in order to force one of the dihedrals in the protein to change
from a trans conformation to a cis conformation.  I am not interested in
finding the free energy of this conformational change, but I used
slow-growth in order to introduce the change in a somewhat uniform way
throughout a small simulation.

During the run using slow growth, I used anisotropic coupling of pressure
(also used in the equilibration).  However, I observe a huge spike in the
system pressure (~0 kJ/mol/nm^3 -> ~4000 kj/mol/nm^3) and
volume (~570nm^3 -> ~770nm^3).  I also see a huge rise in the Total Energy
(-5e-5 -> -1.5e-5).  

After this perturbation, I imposed the original forcefield again and
eventually, the system pressure, volume, and energy went back to "normal." 
I would like someone's opinion on this before I attempt to assign any
physical meaning to this.  Is anything in mdrun turned off when using
slow-growth FEP?  Can anyone think of any reason for this to occur?

Thanks in advance,
David 

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David Bostick					Office: 262 Venable Hall
Dept. of Physics and Astronomy			Phone:  (919)962-0165 
Program in Molecular and Cellular Biophysics 
UNC-Chapel Hill					
CB #3255 Phillips Hall				dbostick at physics.unc.edu	
Chapel Hill, NC 27599	           		http://www.unc.edu/~dbostick	
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