GVVPT2 Multireference Perturbation Theory Description of Diatomic Scandium, Chromium, and Manganese

With relatively simple model spaces derived from valence bond models, a straightforward zero-order Hamiltonian, and the use of moderate-sized Dunning-type correlation consistent basis sets (cc-pVTZ, aug-cc-pVTZ, and cc-pVQZ), the second order generalized Van Vleck perturbation theory (GVVPT2) method is shown to produce potential energy curves (PECs) and spectroscopic constants close to experimental results for both ground and low-lying excited electronic states of Sc2, Cr2 and Mn2. In spite of multiple quasidegeneracies (particularly for the cases of Sc2 and Mn2), the GVVPT2 PECs are smooth with no discontinuities. Since these molecules have been identified as ones that widely used perturbative methods are inadequate for describing well, due to intruder state problems, unless shift parameters are introduced that can obfuscate the physics, this study suggests that the conclusion about the inadequacy of multireference perturbation theory be re-evaluated. The ground state of Sc2 is predicted to be X5∑u –, and its spectroscopic constants are close to the ones at the MRCISD level. Near equilibrium geometries, the 13∑u – electronic state of Sc2 is found to be less stable than the quintet ground state by 0.23 eV. The Cr2 PEC has several features of the Rydberg–Klein–Rees (RKR) experimental curve (e.g., the pronounced shelf at elongated bond lengths), although the predicted bond length is slightly long (R e = 1.80 Å with cc-pVQZ compared to the experimental value of 1.68 Å). The X1∑g + ground state of Mn2 is predicted to be a van der Waals molecule with a long bond length, R e, of 3.83 Å using a cc-pVQZ basis set (experimental value = 3.40 Å) and a binding energy, D e, of only 0.05 eV (experimental value = 0.1 eV). We obtained R e = 3.40 Å and D e = 0.09 eV at the complete basis set (CBS) limit for ground state Mn2. Low lying excited state curves have also been characterized for all three cases (Cr2, Mn2, and Sc2) and show similar mathematical robustness as the ground states. These results suggest that the GVVPT2 multireference perturbation theory method is more broadly applicable than previously documented.