Potential Energy Curves in the CASSCF/CASPT2 and FS-MR-CC Methods: The Role of Relativistic Effects

Abstract

Ab initio CASSCF/CASPT2 calculations for the electronic ground and for a wide range of excited states of Li2 and Na2 dimers are presented. The computed spectroscopic parameters agree very well with the experimental data. This indicates that the old CASSCF/CASPT2 method can be as successfully applied to study excited states of molecules as recently developed the multireference Fock-space coulped-cluster method. The role of relativistic effects in the correct description of the potential energy curves has been investigated using as an example the SiAu molecule. The accuracy of the new infinite-order two-component relativistic method has been studied and its advantage over the Douglas-Kroll-Hess method demonstrated.

Multiconfiguration Pair-Density Functional Theory for Transition Metal Silicide Bond Dissociation Energies, Bond Lengths, and State Orderings

“…For NbSi, the dominant configuration has a weight of only 0.14, with two other configurations with weights above 0. 10. This shows that the ground state wave function of NbSi is more multiconfigurational in nature than VSi and TaSi.…”

“…This substantial increase in experimental accuracy produces benchmark values against which quantum chemistry methods can now be tested, and the accuracy attainable can be a significant indicator of whether the bonding is modelled adequately. This kind of test of theory is particularly interesting because prediction of the energetics of open-shell transition metal compounds is complicated by near-degeneracy effects that produce a plethora of low-lying electronic states [ 10 , 11 ] that can best be modelled with multiconfiguration quantum mechanical methods.…”

Comparison of Methods for Active Orbital Selection in Multiconfigurational Calculations

“…Unlike the isoelectronic H 2 , Li 2 is triplet unstable at its equilibrium geometry and therefore (moderately) strongly correlated. Indeed, high-quality calculations show that the Hartree−Fock reference has only 90% weight in this effectively two-electron wave function, 51 which is a clear sign of nondynamical correlation. Our standard scheme yields an equilibrium distance of 2.722 Å, slightly longer than in highlevel calculations (e.g., 2.68 Å, 51 2.67 Å experiment 52 ), but the potential curve is very shallow.…”

“…Indeed, high-quality calculations show that the Hartree−Fock reference has only 90% weight in this effectively two-electron wave function, 51 which is a clear sign of nondynamical correlation. Our standard scheme yields an equilibrium distance of 2.722 Å, slightly longer than in highlevel calculations (e.g., 2.68 Å, 51 2.67 Å experiment 52 ), but the potential curve is very shallow. The electronic Hessian of the closed-shell Restricted Hartree−Fock (RHF) solution has three negative eigenvalues, the smaller two (in absolute value) of which are degenerate.…”