Description of C₂ dissociation using a naive treatment of dynamical correlation in the presence of quasidegeneracy of varying degree

Multireference perturbation theory with improved virtual orbitals for radicals: More degeneracies, more problems

“…Because our method builds an effective (dressed) Hamiltonian matrix in the reference state basis under the impact of perturbation, and its diagonalization provides the mixing of the nondynamical correlation with the dynamical one (in a target‐specific manner) which allows the relaxation of the reference space component upon constructing the perturbation corrections. As such, the SSMRPT is capable to handle quasi‐degenerate states, such as avoided crossings . The method has already been applied successfully to calculate the low‐lying states of LiF and C 2 , demonstrating the stability and performance of the SSMRPT method for systems in which the nature of the wave function varies as a function of bond length, especially in the region of an avoided crossing.…”

“…As such, the SSMRPT is capable to handle quasi‐degenerate states, such as avoided crossings . The method has already been applied successfully to calculate the low‐lying states of LiF and C 2 , demonstrating the stability and performance of the SSMRPT method for systems in which the nature of the wave function varies as a function of bond length, especially in the region of an avoided crossing. The calculations of the SSMRPT on LiF and C 2 systems illustrate that the performance of it is similar to that of other quasi‐degenerate second‐order MRPT methods suitable for treating avoided crossings/conical intersections.…”

“…Designing methods which can accurately treat MR‐systems at an affordable computational price present a challenge to the practitioners of electronic structure theory . Various workers in the realm of electronic structure theory have suggested different approaches to address these problems . Reviews of various MRPT methods can be found in Ref.…”

“…Recently, we developed an efficient implementation for adapting state‐specific multireference perturbation (SSMRPT) theory based on a multipartitioned Møller‐Plesset (MP) to quantum chemical calculation in which IVOs are exploited to describe static correlation in combination with CAS model via IVO‐CASCI . The resulting method termed as IVO‐SSMRPT borrows the active space concept from MRPTs. It is a very useful tool for its efficacy in encapsulating various types of correlation effects for an accurate treatment of bond stretching or breaking, chemical reaction pathways, and excited states, particularly those dominated by many‐electron transitions.…”

“…Our previous works illustrate that use of IVO‐SSMRPT computations with small reference spaces provides good correlation energies and a balanced description for the structures of molecules and energy surfaces of chemical dissociation processes for the ground and electronically excited states . The primary goal of this study is to investigate the accuracy of the IVO‐SSMRPT protocol for evaluating energy differences, for example, singlet‐triplet (ST) gaps in multiradical systems [which are the prerequisites for the rational design of improved magnetic materials and for fine‐tuning of their properties] along with vibrational frequencies and bond parameters.…”