The magnetic field perturbation method MFP

This theory was first formulated by Stephens (1985). In his approach to go beyond the Bom-Oppenheimer approximation, Stephens mixed excited electronic states with the ground state. Though that approach seemed to call for the difficult calculation of excited electronic states, he finally arrived at expressions, that only involve ground state properties. As an example to the application of the theory Jalkanen et al. (1989b) report nuclear shielding tensors, atomic polar and axial tensors, as well as IR and VCD intensities of the ammonia isotopomer N H (NHDT) using different basis sets. 

The electric dipole transition moment of the i vibrational mode at freqency w, is calculated as  

The magnetic dipole transition moment on the other hand formulates as  

The two transition moments can now be combined in the common way to give the vibrational rotatory strength  

In the expressions above t/)c(Ro H/j) is the ground state electronic wave function at the equilibrium nuclear position Rq, X are the Cartesian displacement coordinates of nucleus A(q = x,y,z), related to the normal coordinates by S ai. 

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