FATAL ERROR: can't use non-numeric floating-point value as operand of "-" - tcl

Question

I am performing SMD simulation using NAMD with TCL. However I get FATAL ERROR: can't use non-numeric floating-point value as operand of "-"; why?
I am doing steered molecular dynamics and the code below is to introduce a virtual spring to produce an external force.I am constraining multiple atom positions for the spring to move towards the position.
The constant velocity method can be used to pull multiple atoms.
Details of the code is below;
# Atoms selected for force application
set id1 [atomid AP1 10 CA]
set grp1 {}
lappend grp1 $id1
set a1 [addgroup $grp1]
set id2 [atomid AP1 14 CA]
set grp2 {}
lappend grp2 $id2
set a2 [addgroup $grp2]
set id3 [atomid AP1 19 CA]
set grp3 {}
lappend grp3 $id3
set a3 [addgroup $grp3]
set id4 [atomid AP1 22 CA]
set grp4 {}
lappend grp2 $id4
set a4 [addgroup $grp4]
set id5 [atomid AP1 25 CA]
set grp5 {}
lappend grp5 $id5
set a5 [addgroup $grp5]
set id6 [atomid AP1 28 CA]
set grp6 {}
lappend grp6 $id6
set a6 [addgroup $grp6]
set id7 [atomid AP1 32 CA]
set grp7 {}
lappend grp7 $id7
set a7 [addgroup $grp7]
set id8 [atomid AP1 36 CA]
set grp8 {}
lappend grp8 $id8
set a8 [addgroup $grp8]
set id9 [atomid AP1 42 CA]
set grp9 {}
lappend grp9 $id9
set a9 [addgroup $grp9]
# set the output frequency, initialize the time counter
set Tclfreq 50
set t 0
# contraint points
set c1x -19.450
set c1y -3.791
set c1z 12.790
set c2x -7.811
set c2y -8.997
set c2z 11.372
set c3x 9.267
set c3y -12.502
set c3z 13.258
set c4x 16.102
set c4y -20.086
set c4z 11.251
set c5x 24.674
set c5y -13.698
set c5z 9.676
set c6x 22.147
set c6y -3.551
set c6z 13.986
set c7x 8.271
set c7y -2.899
set c7z 18.747
set c8x 8.827
set c8y 8.698
set c8z 15.017
set c9x 22.774
set c9y 1.488
set c9z 16.176
# force constant (kcal/mol/A^2) #7.2
set k 1.0
# pulling velocity (A/timestep)
set v 0.002
set outfilename da_smd_tcl.out
open $outfilename w
proc calcforces {} {
global Tclfreq t k v a1 a2 a3 a4 a5 a6 a7 a8 a9 c1x c1y c1z c2x c2y c2z c3x c3y c3z c4x c4y c4z c5x c5y c5z c6x c6y c6z c7x c7y c7z c8x c8y c8z c9x c9y c9z outfilename
# get coordinates
loadcoords coordinate
set r1 $coordinate($a1)
set r1x [lindex $r1 0]
set r1y [lindex $r1 1]
set r1z [lindex $r1 2]
set r2 $coordinate($a2)
set r2x [lindex $r2 0]
set r2y [lindex $r2 1]
set r2z [lindex $r2 2]
set r3 $coordinate($a3)
set r3x [lindex $r3 0]
set r3y [lindex $r3 1]
set r3z [lindex $r3 2]
set r4 $coordinate($a4)
set r4x [lindex $r4 0]
set r4y [lindex $r4 1]
set r4z [lindex $r4 2]
set r5 $coordinate($a5)
set r5x [lindex $r5 0]
set r5y [lindex $r5 1]
set r5z [lindex $r5 2]
set r6 $coordinate($a6)
set r6x [lindex $r6 0]
set r6y [lindex $r6 1]
set r6z [lindex $r6 2]
set r7 $coordinate($a7)
set r7x [lindex $r7 0]
set r7y [lindex $r7 1]
set r7z [lindex $r7 2]
set r8 $coordinate($a8)
set r8x [lindex $r8 0]
set r8y [lindex $r8 1]
set r8z [lindex $r8 2]
set r9 $coordinate($a9)
set r9x [lindex $r9 0]
set r9y [lindex $r9 1]
set r9z [lindex $r9 2]
# calculate forces
set f1x [expr $k*($c1x-$r1x)]
set f1y [expr $k*($c1y-$r1y)]
set f1z [expr $k*($c1z+$v*$t-$r1z)]
lappend f1 $f1x $f1y $f1z
set f2x [expr $k*($c2x-$r2x)]
set f2y [expr $k*($c2y-$r2y)]
set f2z [expr $k*($c2z+$v*$t-$r2z)]
lappend f2 $f2x $f2y $f2z
set f3x [expr $k*($c3x-$r3x)]
set f3y [expr $k*($c3y-$r3y)]
set f3z [expr $k*($c3z+$v*$t-$r3z)]
lappend f3 $f3x $f3y $f3z
set f4x [expr $k*($c4x-$r4x)]
set f4y [expr $k*($c4y-$r4y)]
set f4z [expr $k*($c4z+$v*$t-$r4z)]
lappend f4 $f4x $f4y $f4z
set f5x [expr $k*($c5x-$r5x)]
set f5y [expr $k*($c5y-$r5y)]
set f5z [expr $k*($c5z+$v*$t-$r5z)]
lappend f5 $f5x $f5y $f5z
set f6x [expr $k*($c6x-$r6x)]
set f6y [expr $k*($c6y-$r6y)]
set f6z [expr $k*($c6z+$v*$t-$r6z)]
lappend f6 $f6x $f6y $f6z
set f7x [expr $k*($c7x-$r7x)]
set f7y [expr $k*($c7y-$r7y)]
set f7z [expr $k*($c7z+$v*$t-$r7z)]
lappend f7 $f7x $f7y $f7z
set f8x [expr $k*($c8x-$r8x)]
set f8y [expr $k*($c8y-$r8y)]
set f8z [expr $k*($c8z+$v*$t-$r8z)]
lappend f8 $f8x $f8y $f8z
set f9x [expr $k*($c9x-$r9x)]
set f9y [expr $k*($c9y-$r9y)]
set f9z [expr $k*($c9z+$v*$t-$r9z)]
lappend f9 $f9x $f9y $f9z
# apply forces
addforce $a1 $f1
addforce $a2 $f2
addforce $a3 $f3
addforce $a4 $f4
addforce $a5 $f5
addforce $a6 $f6
addforce $a7 $f7
addforce $a8 $f8
addforce $a9 $f9
# output
set foo [expr $t % $Tclfreq]
if { $foo == 0 } {
set outfile [open $outfilename a]
set time [expr $t*2/1000.0]
puts $outfile "$time $r2z $f2z"
close $outfile
}
incr t
return
}

Answer 1

There's something unexpected (non-numeric) in one of your variables.
Which one is the tricky bit. You don't say which line has the error (the errorInfo global will contain this) and because you don't put your expressions in braces, even the errorInfo will be not 100% informative. (Putting expressions in braces allows Tcl to compile the expressions — making them faster and safer — and makes the error message on a bad value rather more informative as the runtime engine has more information available.)
Given what error message you're receiving, it's probably not an empty string.
As a side note, you can shorten sequences like this:
set r1 $coordinate($a1)
set r1x [lindex $r1 0]
set r1y [lindex $r1 1]
set r1z [lindex $r1 2]
to this:
lassign $coordinate($a1) r1x r1y r1z