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Calculation Of Diagonal Born-Oppenheimer CorrectionsCorrections 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 {$\Delta E_{DBOC} = \langle \Psi_{el} | \sum_A - \frac{\hbar^2}{2 M_A} \nabla_A^2 | \Psi_{el}\rangle $} 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. theoretical details to DBOC corrections implementational details to DBOC corrections The DBOC is calculated by simply invoking the keyword DBOC=ON 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. Examples |