A simplified ab initio treatment of diradicaloid structures produced from stretching and breaking chemical bonds

Abstract

The present investigation reports on the prospect of using state specific multireference perturbation theory (SSMRPT) with an improved virtual orbital complete active space configuration interaction (IVO-CASCI) reference function (IVO-SSMRPT) to generate potential energy surfaces (PESs) for molecular systems [such as CH, CH, CH, HO, LiH, and KN] by stretching and breaking of suitable bonds with modest basis sets. We have also revisited the dissociation energy profile of triplet ketene which exhibits a step-like structure in the observed rate. The application of the method has also been made to the ionization energies of HO. Although the perturbative corrections are obtained by the diagonalization of the effective Hamiltonian, in IVO-SSMRPT, only one physically relevant solution is achievable. It is parameter free and does not require any threshold to avoid the intruder problem. It is strictly size-extensive and size-consistent provided that local orbitals are used. The PESs obtained with our approach are smooth all along the reaction path. Our estimates are in close agreement with the available reference data indicating that IVO-SSMRPT is a robust paradigm for the accurate computation of ground, excited and ionized states as it captures the mutual inter-play of different flavors of correlation effects in a balanced and accurate way.

Simplified Treatment of Electronic Structures of the Lowest Singlet and Triplet States of Didehydropyrazines

“…Initially, the optimized geometries generated from the IVO-CASCI­(8,8)/cc-pVTZ calculations have been employed as an input to generate the CASSCF­(8,8) orbitals (described in Figure ). A number of recent studies have shown that, in general, the SSMRPT with both CASSCF and IVO-CASCI orbitals supply very similar estimates, provided that the reference function is constructed appropriately. ,,,− The ordering of the lowest lying singlet and triplet states provided by CASSCF-SSMRPT­(8,8)/cc-pVTZ for didehydropyrazines agrees with IVO-SSMRPT­(8,8) and MR-AQCC­(12,10)/cc-pVTZ findings. The notable difference between the IVO-SSMRPT/cc-pVTZ and CASSCF-SSMRPT/cc-pVTZ results is found in the ST energy gap of the para (2,5) isomer (with deviations of 6.9 kcal/mol).…”

“…Even in the case of CAS­(2,2), each cluster operator entails not only one- and two-body cluster operators, but also a subset of three- and four-body cluster operators, which describe singles–doubles excitations from other reference configuration. Note that the IVO-SSMRPT usually eliminates most of the erroneous predictions of CCSD protocols. ,,,− …”

“…Chattopadhyay et al already showed that IVO-SSMRPT, even with small, and systematically and intuitively defined CAS, is competitive in accuracy with widely used MRPTs (say MRMP2, CASPT2, NEVPT2, etc.) and MRCC methods for a variety of cases, including bond dissociations and energy differences between multiplets. ,,,− …”

“…It is worth emphasizing here that a critical assessment of any MR protocol tailored to handle electronic states in state-specific manner is to describe the excited states with a similar reliability and ease as is applicable for the ground state. Previous studies have led to the conclusion that the convergence of the IVO-SSMRPT working equations for the excited electronic states is slower in comparison to the ground state, akin to other single-root MR methods. ,,,− Thus, care and caution needs to be exercised while performing single-root MR calculations for excited states of radicaloid systems.…”

“…Many variants of MRPT methods have been suggested and widely used (for a general overview, see, e.g., ref ). The IVO-SSMRPT method has been successfully implemented for computing energy surfaces and properties of spectroscopic interest. ,,,− Our previous calculations have also been demonstrated that IVO-SSMRPT can describe diradicals accurately ,,, with even the minimal active space. IVO-SSMRPT is a useful alternative to the widely used MRPT computations ,− with various attractive features (vide infra).…”