ABSTRACT
In section 2.4, the classical expression for the energy of a molecule consisting of l particles, nuclei and electrons, was written down using XYZ axes. The XYZ axes have both origin and orientation fixed in space. In section 2.5, the translational energy was separated by introducing XYZ axes parallel to the XYZ axes but with their origin at the molecular centre of mass. The XYZ axes thus have spacefixed orientation but a molecule-fixed origin. After separating out the translation, the internal (rovibronic) energy is expressed using the (Xr, Yr, Zr ) coordinates of the particles but now there are three constraints on these coordinates [see equation (2.51)]: () ∑ r = 1 ι m r X r = ∑ r = 1 ι m r Y r = ∑ r = 1 ι m r Z r = 0 . https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315273334/90d251cd-96eb-444c-8581-8842f96b239c/content/eq369.tif"/>
