Demo entry 6768199

center of mass


Submitted by anonymous on Nov 02, 2018 at 07:03
Language: Tcl. Code size: 1.1 kB.

proc center_of_mass {selection} {
        # some error checking
        if {[$selection num] <= 0} {
                error "center_of_mass: needs a selection with atoms"
        # set the center of mass to 0
        set com [veczero]
        # set the total mass to 0
        set mass 0
        # [$selection get {x y z}] returns the coordinates {x y z} 
        # [$selection get {mass}] returns the masses
        # so the following says "for each pair of {coordinates} and masses,
    #  do the computation ..."
        foreach coord [$selection get {x y z}] m [$selection get mass] {
           # sum of the masses
           set mass [expr $mass + $m]
           # sum up the product of mass and coordinate
           set com [vecadd $com [vecscale $m $coord]]
        # and scale by the inverse of the number of atoms
        if {$mass == 0} {
                error "center_of_mass: total mass is zero"
        # The "1.0" can't be "1", since otherwise integer division is done
        return [vecscale [expr 1.0/$mass] $com]

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