Calculating the center of mass of a molecule
 
To calculate the center of mass I have written the following Excel routine for Excel. I noticed that when doing the calculation in Excel itself, using the normal Excel formulae, that errors started to kreep in due to rounding errors. I think Excel calculates with single precision numbers. The routine below uses double precision.
 
Excel VBA code to calculate the center of mass. The function can be called as:
=center_of_mass(C11:C30,B11:B30)
 
The first range of cells (in my example C11:C30) must contain the coordinates, for example all x-coordinates. The second range of cells (in my example B11:B30) must contain the atom masses. The routine is actually just doing a weighted average of the coordinates using the masses of the atoms as the weights in the averaging process.
 
You need to call the function for each coordinate: once for the x-coordinate, once for the y-coordinate and finally once for the z-coordinate. The resulting set of values for x,y,z is the center of mass of the molecule.
 
Code:

Public Function center_of_mass(ByVal Values As Variant, ByVal Weights As Variant) As Double
  Dim dbl As Double
  Dim dblTotalWeight As Double
  Dim dblValue As Double
  Dim dblWeight As Double
  Dim lng As Long
  
  For lng = 1 To Values.Count
    dbl = dbl + CDbl(Values(lng).Value) * CDbl(Weights(lng).Value)
    dblTotalWeight = dblTotalWeight + CDbl(Weights(lng).Value)
  Next lng
  center_of_mass = dbl / dblTotalWeight
End Function