Relativistic state-specific multireference perturbation theory incorporating improved virtual orbitals: Application to the ground state single-bond dissociation

Abstract

Using four-component (4c) relativistic spinors, we present a computationally economical relativistic ab initio method for molecular systems employing our recently proposed second-order state-specific multireference perturbation theory (SSMRPT) incorporating the improved virtual orbital-complete active space configuration interaction (IVO-CASCI) reference wavefunction. The resulting method, 4c-IVO-SSMRPT [calculate one state at a time] is tested in pilot calculations on the homonuclear dimers including Li(2), Na(2), K(2), Rb(2), F(2), Cl(2), and Br(2) through the computations of the ground state potential energy curves (PECs). As SSMRPT curbs intruder effects, 4c-IVO-SSMRPT is numerically stable. To our knowledge, the SSMRPT in the 4c relativistic framework has not been explored in the past. Selective spectroscopic constants that are closely related to the correct shape and accuracy of the energy surfaces have been extracted from the computed PECs. For the halogen molecules, a relativistic destabilization of the bond has been found. Relativistic and electron correlation effects need to be incorporated to get reliable estimates. Our results are in good accordance with reference theoretical and experimental data which manifests the computational accuracy and efficiency of the new 4c-IVO-SSMRPT method. The method opens for an improved description of MR systems containing heavy elements. The inexpensiveness of IVO-CASCI makes 4c-IVO-SSMRPT method promising for studies on large systems of heavy elements.

Taming the Electronic Structure of Diradicals through the Window of Computationally Cost Effective Multireference Perturbation Theory

“…Thus, the SSMRPT method encompasses almost all the desirable properties that a MR method should have and thus it seems worthwhile to investigate its performance for numerous systems as is evident from the published works 32,[34][35][36][37][38] . The method has also been employed for studying dissociation surfaces for electronic states of molecules containing medium heavy atoms 39 .…”

State‐specific multireference perturbation theory: development and present status

“…The combined application of EPCISO (effective and polarized CI spin‐orbit) and SO‐CASPT2 is also useful to investigate heavy‐element systems. Very recently, our group developed the relativistic SSMRPT for the four‐component Dirac‐Coulomb Hamiltonian employing MP partitioning of the Hamiltonian and IVO‐CASCI as the reference function rather than the CASSCF one (4c‐IVO‐SSMRPT) . To our knowledge, this is the first relativistic implementation of the SSMRPT method.…”

“…Inspection of the table illustrates that the agreement between the 4c‐IVO‐SSMRPT/Dyall‐v3z and the computationally expensive full‐blown DEA‐IH‐FS‐CCSD/[14s14p7d6f] values is very good with a difference of Δω e = 6 cm − 1 , ΔD e = 0.7 eV, and ΔR e = 0.1 Å. Pilot applications indicate that the 4c‐SSMRPT model will prove useful in many cases where simultaneous consideration of relativistic and electron correlation effects in a balanced manner is necessary.…”

“…To confirm the applicability of the 4c‐IVO‐SSMRPT method, next we consider the computation of dissociation process of the ground state Rb 2 . Figure describes the PESs for the ground state of the Rb 2 molecule.…”

“…Note that the energy value at the equilibrium internuclear distance is shifted to zero for all approaches. Reproduced from Ref . Copyright 2015 Wiley Periodicals, Inc…”