[gmx-users] dg/dl unit

[Michael Shirts]

> 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.

Here's my naive question- is there a legitimate reason to do a slow
growth simulation, other than in the context of averaging a number of
slow growth simulations, a la Jarzynski?  It's a completely
uncontrolled approximation, because the ensemble always lags behind
the Hamiltonian in a way that cannot be corrected for.  Shouldn't we
all be doing fixed-lambda sampling, or (when I get my fixes in),
WHAM/Bennett acceptance ratio methods?

I suppose one could do slower and slower growth until it converges,
but for anything but the smallest changes (changing just a couple of
atom types, no disappearance/appearance of atoms), its would be very
inefficient.

Best,
Michael