[gmx-users] dg/dl unit

Chris Oostenbrink c.oostenbrink at few.vu.nl
Thu Nov 3 10:54:53 CET 2005

>>I am just wondering what the units for dg/dl mean in a free energy 
>>perturbation run. I get a graph of dG/dl (KJmol-1nm-2l-1) vs time (ps). 
>>Now, ideally i want KJmol-1lambda-1 vs lambda graph so that i can integrate 
>>it to get back a value that is KJ/mol.
>>I am just wondering what (nm-2l-1) unit means and if it is related to ps as 
>>i suspect.
>This was an error, it should be KJmol-1lambda-1 (the data is correct).
>This has been corrected in Gromacs 3.3.
Can I ask a naive question? Since when is lambda a unit? lambda is a 
unitless quantity and quantities usually do not appear in a unit. If you 
want to do it completely correct, I think that dG/dl has the unit kJmol-1.

If you would do a slow growth simulation in which you modify lambda as a 
function of time, then you probably write out dG/dl * dl/dtime and the 
unit would be kJmol-1ps-1.


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