VMD User's Guide

The decimal in ”1.0” is important since otherwise Tcl does integer division. Finally, this value isused to scale the sum of the coordinate vectors (with vecscale), which returns the new value, whichis itself returned as the result of the procedure.The center of mass function is slightly harder because you have to get the mass as well asthe x, y, z values, then break that up into to components. The formula for the center of mass is∑ mi x i / ∑ mass iproc center_of_mass {selection} {# some error checkingif {[$selection num] center_of_mass $selInfo) 0.912778 4.61792 2.78021The opposite of ”get” is ”set”. Many keywords (most notably, ”x”, ”y”, and ”z”) can be set tonew values. This allows, for instance, atom coordinates to be changed, the occupancy values to beupdated, or user forces to be added. You can also change the resname, segid, and so forth, whichmay be easier to do within VMD than, for example, editing a PDB file by hand.set sel [atomselect top "index 5"]$sel get {x y z}{1.450000 0.000000 0.000000}$set set {x y z} {{1.6 0 0}}159