State‐specific multireference perturbation theory with improved virtual orbitals: Taming the ground state of F₂, Be_2, and N₂

Abstract

Adaptation of improved virtual orbitals (IVOs) in state-specific multireference perturbation theory using Møller-Plesset multipartitioning of the Hamiltonian (IVO-SSMRPT) is examined in which the IVO-complete active space configuration interaction (CASCI) is used as an inexpensive alternative to the more involved CAS-self-consistent field (CASSCF) orbitals. Unlike the CASSCF approach, IVO-CASCI does not bear tedious and costly iterations beyond those in the initial SCF calculation. The IVO-SSMRPT is intruder-free, and explicitly size-extensive. In the present preliminary study, the IVO-SSMRPT method which relies on a small reference space is applied to study potential energy surfaces (PES) of the ground state of challenging, multiconfigurational F2 , Be2 , and N2 molecules. These systems provide a serious challenge to any ab initio methodology due to the presence of an intricate interplay of nondynamical and dynamical correlations to the entire PES. The quality of the computed PES has been judged by extracting spectroscopic parameters and vibrational levels. The reported results illustrate that the IVO-SSMRPT method has a potential to yield accuracies as good as the CASSCF-SSMRPT one with reduced computational labor. Even with small reference spaces, our estimates demonstrate a good agreement with the available experimental values, and some benchmark computations. The blend of accuracy and low computational cost of IVO-SSMRPT should deserve future attention for the accurate treatment of electronic states of small to large molecular systems for which the wavefunction is characterized by various configurations.

State‐specific multireference perturbation theory: development and present status

“…It is pertinent to mention that Hirao and his collaborators have also proposed a similar MRPT2 approach with CASCI (denoted as CASCI‐MRMP2). In contrast to IVO‐SSMRPT, CASCI or IVO‐CASCI based MCQDPT and its single‐root variant, MRMP2 of Hirao are size‐inextensive. At this juncture, we should note that even though the IVO‐SSMRPT is based on the IVO‐CASCI wave function, which is known to be invariant with respect to a unitary transformation of the active orbitals, this property does not extend to the IVO‐SSMRPT (and CASSCF‐SSMRPT) approach.…”

“…A practical concern with this approach is the arbitrariness of the orbital from which an electron is removed to generate the Fock operator used to construct low occupancy orbitals. The IVO‐CASCI method has been adopted successfully in the MRMP2/MCQDPT and in the state‐specific MRPT (SSMRPT) by our group in very recently. The blend of accuracy at a fraction of the computational labor offered by the IVO‐CASCI‐based MCQDPT and SSMRPT methods demonstrates that the method may open a practical and robust tool for studying MR situations with various levels of complexity in the near future.…”

“…The calculated error energy surfaces for the ground state F 2 is depicted in Figure (for detail see Ref ). The results for CR‐CCSD(T), SS‐MRCCSD, CASPT2, and MRCI calculations have been taken from the recent work of Yang et al For this comparison, we have used the same scheme as that of Yang et al .…”

“…The IVO‐SSMRPT and SSMRPT results for 22 vibrational levels in terms of deviations with respected to the experimental values are depicted in Figure . For detail, one can consult Table 3 of Ref along with the corresponding discussion. Closeness of the performance of the IVO‐SSMRPT and CASSCF‐SSMRPT is also visible in Figure .…”

“…] methods from the FCI values for bond breaking in the ground state F 2 [CAS(2,2) reference wave function with cc‐pVDZ basis set has been used in all MR‐based calculations considered here]. Reproduced from Ref . Copyright 2015 Wiley Periodicals, Inc.…”