compute ti at l=0

Dear lammps users,

I have recently started using the fix adapt and compute ti commands in
lammps, and had some queries.

Here is my input file. It is intended to tether particles of type 1 to
type 2 using the gauss pair interaction. The gaussian energy is
"switched on" slowly from 0 to 1. Type 1 particles interact via a
truncated LJ potential.

As the first step, I would like to compute the dU/dl for l=0

U_s=-lambda U_g
dU_s/dl = -U_g

df/dl = -1.0

Unfortunately I get all zeros from the compute ti.
Is this an expected behavior ?

According to the relations above, the value of the derivative would be
the value of the original gauss potential computed for the ensemble

The best person to answer this is Sai (CCd).
He's in the middle of a vacation and moving to
a new position at MIT, so not sure how quickly
he will see this.


The derivative at lambda=0 return all 0's to avoid undefined calculations.
dU(lambda)/dlambda = U_unscaled*f'(lambda)/f(lambda)
will be undefined when f(lambda)=0. Use a small f(lambda) (0.01 or smaller) to get around this issue.


Thank you, Sai. I had indeed tried small values - and they work well.

Perhaps a comment/note in the compute ti manual page might be added -
as a pointer to beginners? Functions like (1-lambda) will also give
zeros at the boundary...

Manish Agarwal
<[email protected]...>
Postdoctoral Research Scientist
Chemical Engineering
Columbia University
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