Ab Initio Multireference Investigation of Disjoint Diradicals: Singlet versus Triplet Ground States

Abstract

We present improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO-CASCI) and IVO-CASCI-based multireference Møller-Plesset perturbation theory (MRMPPT) calculations with an aim to elucidate the electronic structure of tetramethyleneethane (TME) in its lowest singlet and triplet state and to quantify their order and extent of splitting. The potential surfaces of singlet and triplet states for the twisting of TME are also studied. We found that the triplet state is higher in energy than the singlet one in the whole range of twisting angles with the energy gap minimum at a twisting angle of about 45°. Harmonic vibrational frequencies of TME have also been calculated for both the states. We also report the ground to first excited triplet state transition energies. Our results are analyzed with respect to the results available in the literature to illustrate the efficacy of our methods employed. We also demonstrate that the spin character of the ground state of disjoint, TME-like diradicals can be manipulated by using appropriate selection of annulenic spacer to separate the allyl groups of TME.

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

“…VMC calculations based on CASSCF(6,6)/cc‐pVTZ geometries for twisting potential delineated a possible near‐degeneracy between the singlet and triplet states that disappeared at the DMC level of calculation (using a truncated cc‐pV5Z basis coupled with a pseudopotential to create a torsional potential), yielding a small gap between the states with the singlet 1 A state lower in energy at all torsional angles . Note that our previous IVO‐MRPT2 calculations explicitly revealed 1 [TME] to be more stable than 3 [TME] and gave the 90° twisted structure as the minimum of the singlet state . There is no tiny hump on the computed torsion surface for the single state, however, the minimum in the TPS of 3 [TME] occurs at a twisted angle near 45° .…”

“…Note that our previous IVO‐MRPT2 calculations explicitly revealed 1 [TME] to be more stable than 3 [TME] and gave the 90° twisted structure as the minimum of the singlet state . There is no tiny hump on the computed torsion surface for the single state, however, the minimum in the TPS of 3 [TME] occurs at a twisted angle near 45° . Although MRCCSD(T)/cc‐pVTZ ′ with CASSCF(2,2) orbitals calculations provide very flat potential energy surface for 1 [TME], one tiny minimum is found nearly at 30 ° and two maxima are clearly visible at 0° and 90° in the 1 [TME].…”

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

“…Our results have been compared with state-of-the-art results and experimental data, if available. The IVO-CASCI has already been used in the past to study radicaloids52,53 . It should be noted that the prime aim of the work is not to answer the still open questions of interpretation of electronic structure of the systems investigated but rather to judge the efficacy of the newly developed IVO-SSMRPT to provide a viable means of calculation of singlet and non-singlet states of radicals.…”

State‐specific multireference perturbation theory: development and present status

“…This observation is in accord with the estimations of high‐level quantum chemical methods. Investigations by our group indicate that the geometrical parameters provided by SSMRPT calculations are in well agreement with the IVO‐MRMP method. The SSMRPT geometries for TDB are also in acceptable agreement with the spin‐adapted CCSD(T) [RCCSD(T)] (see Figures 3 and 4 in Ref ).…”

“…For these systems, CAS(2,2) is used as references wave function in the calculations of SSMRPT method. Details of the SSMRPT calculations have been given in Refs .…”