Born-Oppenheimer separation breakdown

These maxima correspond to poles near the real R axis in a proper analytic continuation of P(R), and mark the breakdown of the Born-Oppenheimer separation. 

Laser excitation is not only a means for providing the necessary energy for chemistry to take place. We begin the discussion of the selectivity achievable by the excitation and the specificity of the detection for the special case of diatomic molecules. Then the spectrum is simpler in that there is only one vibrational mode as well as only one rotational constant. But once the molecule is electronically excited we will find it necessary to allow for the breakdown of the Born-Oppenheimer separation. 

All of the calculations have been performed at the experimental equilibrium distance R = 1.128 A, in order to enable a proper comparison with the EOM-CCSD reference. In so far as there are neither largely interacting excited states nor special reasons for expecting a breakdown of the Born Oppenheimer approximation, great changes in the MAE are not expected if one takes the (SC) SDCI ground state equilibrium value for Re which is Re = 1.140 A (very close to the CCSD value, as expected Cfr. table 1). We have performed a separate calculation of the whole set of VEE with the aug-cc-pVDZ basis set at the Rg distance, in any case. The results have not been included in table II for the sake of clarity, but the total MAE values where 2.34 eV for the MR-SDCI and 0.17 eV for (SC)2mR-SDCI. 

The second problem that often occurs in open-shell van der Waals molecules is the breakdown of the Born-Oppenheimer (BO) approximation. As is well known, the BO approximation can be trusted when the PESs are well separated in energy. However, when certain points on the PES are degenerate this condition is not fulfilled, not in the degenerate points themselves, but also not in nearby points. This breakdown of the BO approximation can be shown as follows. Let us write R for the collection of nuclear coordinates and r for the electron coordinates. Indicating electronic and nuclear interactions by subscripts e and n, respectively, the Schrodinger equation takes the form... 

Rotational constants, centrifugal distortion constants, rotation-vibration interaction constants, Dunham energy coefficients, parameters of the breakdown of the Born-Oppenheimer approximation, and equilibrium intemnclear separations... 

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