Diagonal Born-Oppenheimer correction DBOC

For the majority of systems the Born-Oppenheimer approximation introduces only very small errors. The diagonal Born-Oppenheimer correction (DBOC) can be evaluated relatively easy, as it is just the second derivative of the electronic wave function with respect to the nuclear coordinates, and is therefore closely related to the nuclear gradient and second derivative of the energy (Section 10.8). 

Diagonal Born-Oppenheimer correction (DBOC) for RHF, ROHF, UHF, and Cl wave functions. 

MVDl = one-electron mass-velocity plus Darwin D2 = two-electron Darwin DBOC = diagonal Born-Oppenheimer correction. Please see text for details, rii and are the stretching, 112 is the bending quantum number. 

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