Help with minimization

Dear Users

Im trying to minimize a system which has two parts a nanotube and a helix of nucleotides which is non-covalently bound to the nanotube. Im using Conjugate gradient minimization with a force tolerance of 1.0-8 . When I minimize the whole system it terminates after convergence criteria has been met. but If I fix the helix using fix setforce after the initial convergance and minimize again, it keeps on minimizing and the system reaches a lower energy than without the constraint. Im using lj/cut/coul/cut with a 32.0 Angstrom cut-off. I dont understand why with the constraints the system reaches a lower minima than without the constraints.

Thanks

Milinda Samaraweera
Computational materials chemistry and Computer programming
University of Connecticut
Department of Chemistry
55 N Eagleville road
unit 3060
Storrs CT
USA