AMBER Archive (2005)

Subject: AMBER: reverse TI charge perturbation

From: Eric Hu (yhu_2003_at_yahoo.com)
Date: Fri Feb 11 2005 - 19:59:10 CST

Hi, I am trying to conduct a ligand-only TI energy
perturbation using sander (Amber 8.0) based on prof.
Case's tutorial. I used the same forward input files
for the reverse calculations.
As I understand for "reverse perturbation" I assinged
the inintial charges to be zero on all ligand atoms
and perturbed into normal charges at lambda = 1. First
of all the energy remains positive in vacume after 500
NCYC and 1000 MAXCYC comparing to the normal case
(charged --> uncharged) whose energy drops quickly.
Therefore it is easy to understand why the following
md blows out.
For the similar calculation in water, I first
minimized the water with initial restraints on the
ligand. Then I assign the initial dummy atom types at
lambda = 0 and perturb into normal ones at the end.
However the dv/dl term remains zero throughout the
simulations.

Thank you for any suggestions.

Eric

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