Calculation Of Diagonal Born-Oppenheimer Corrections
Corrections for making the Born-Oppenheimer approximations in the quantum chemical treatment of atoms and molecules are easily computed using perturbation theory. The lowest-order correction is obtained in first-order and takes the form
This correction is known as the diabatic Born-Oppenheimer correction (DBOC) and its inclusion leads to the so-called adiabatic approximation with mass-dependent potentials that are different for the various isotopologues of a molecule.
The DBOC is calculated by simply invoking the keyword
Note that CFOUR calculates the DBOC using nuclear masses.
Note that DBOC can be computed at the HF-SCF, MP1, MP2 as well as the CCSD level for both RHF and UHF reference functions. ROHF may only be used at the HF-SCF level.
Note that geometry optimizations including DBOC can be carried out using numerically evaluated gradients via METHOD=ENERONLY.