#Calculate tilt angle between the longest principal axies of selected atoms and Z-axis
# a.b/(|a||b|)=cos(theta), since VMD returns normalized vector, and Z unit-vector is (0,0,1), the left-hand side is simply the Z component of the longest principal axis, which is the 3rd element of the three principal axes, and which is the 2nd list-element returned by "measure inertia" command