# [gmx-users] Lambda Dependence when delta_lambda = 0

Ilya Chorny ichorny at maxwell.compbio.ucsf.edu
Thu Sep 11 18:52:01 CEST 2003

```I have delta_lambda and init_lambda both set to zero.

Ilya

On Thu, 2003-09-11 at 01:40, David van der Spoel wrote:
> On Thu, 2003-09-11 at 08:39, Ilya Chorny wrote:
> > I read that in the manual but that make sense if you incriment lambda. What
> > happens if delta_lambda = 0.
> > Am I correct to say dh/dl = dH if delta_lambda = 0.
> >
> No, it's the derivative at the lambda value you specify in init_lambda
> dlambda just means how much lambda should change per timestep (or per
> ps)
>
> > Ilya
> >
> >
> >
> > -----Original Message-----
> > Behalf Of David
> > Sent: Wednesday, September 10, 2003 11:34 PM
> > To: gmx-users at gromacs.org
> > Subject: Re: [gmx-users] Lambda Dependence when delta_lambda = 0
> >
> >
> > On Thu, 2003-09-11 at 00:53, Ilya Chorny wrote:
> > > Hi Erik,
> > >
> > > I ran a short test trajectory where 100 ps calculating the free energy
> > > of going from Ar - LJ1 (argon with a sigma one Angstrom smaller). In the
> > > mdp file I set free energy to yes and kept the rest of the free energy
> > > parameters as defaults. I'm not sure I understand what the dH/dl in the
> > > case when delta_lambda = 0. Should it not just be the difference in the
> > > Hamiltonians between Ar and LJ1. The numbers seem a little high (~7000
> > > kJ/mol) and I would expect them to be negative.
> > >
> >
> > dH/dl is the derivative of the energy with respect to lambda, it may
> > very well be non zero for any value of lambda. I think it is described
> > in the manual..
> >
> > > Thanks,
> > >
> > > Ilya
> > >
> > >
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> > --
> > Groeten, David.
> > ________________________________________________________________________
> > Dr. David van der Spoel, 	Dept. of Cell and Molecular Biology
> > Husargatan 3, Box 596,  	75124 Uppsala, Sweden
> > phone:	46 18 471 4205		fax: 46 18 511 755
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