From: Elena Lilkova (
Date: Tue Feb 10 2015 - 08:08:18 CST

Dear FFTK experts,

I am trying to parameterize sulfamic acid anion (NH2SO3-) using FFTK. I
want to put all the atoms in one charge group. But when I try to run a
charge optimization with this charge group I get the following error:

can't use empty string as operand of "-"
can't use empty string as operand of "-"
    while executing
"expr {[lindex $y 1]-$T*log(rand())}"
    (procedure "simulated_annealing" line 30)
    invoked from within
"simulated_annealing $T "
    (procedure "::Optimize::Opt0::handle" line 202)
    invoked from within
"$opt start"
    (procedure "::ForceFieldToolKit::ChargeOpt::optimize" line 424)
    invoked from within
    (procedure "::ForceFieldToolKit::gui::coptRunOpt" line 55)
    invoked from within
"::ForceFieldToolKit::gui::coptRunOpt "
    invoked from within
".fftk_gui.hlf.nb.chargeopt.runOpt invoke "
    invoked from within
".fftk_gui.hlf.nb.chargeopt.runOpt instate {pressed !disabled} {
.fftk_gui.hlf.nb.chargeopt.runOpt state !pressed;
.fftk_gui.hlf.nb.chargeopt.runOpt in..."
    (command bound to event)

When I split the molecule in 2 or more charge groups, the optimization
starts normally.

Elena Lilkova

PhD student,
Atomic Physics Department,
Faculty of Physics,
University of Sofia "St. Kliment Ohridski"

5 James Bourchier Blvd.
1164 Sofia,