An extended multireference study of the electronic states of para-benzyne

Abstract

A state-averaged, multireference complete active space (CAS) approach was used for the determination of the vertical excitation energies of valence and Rydberg states of para-benzyne. Orbitals were generated with a 10- and 32-state averaged multiconfigurational self-consistent field approach. Electron correlation was included using multireference configuration interaction with singles and doubles, including the Pople correction for size extensivity, multireference averaged quadratic coupled cluster (MR-AQCC), and MR-AQCC based on linear response theory. There is a very high density of electronic states in this diradical system-there are more than 17 states within 7 eV of the ground state including two 3s Rydberg states. All excitations, except 2 (1)A(g), are from the pi system to the sigmasigma(*) system. Of the 32 states characterized, 15 were multiconfigurational, including the ground (1)A(g) state, providing further evidence for the necessity of a multireference approach for p-benzyne. The vertical singlet-triplet splitting was also characterized using a two-state averaged approach. A CAS(2,2) calculation was shown to be inadequate due to interaction with the pi orbitals.

The Fractional Occupation Number Weighted Density as a Versatile Analysis Tool for Molecules with a Complicated Electronic Structure

“…The weight of the reference wavefunction is 0.65, and the weight of the doubly excited CSF is 0.27. A number of previous studies used a MCSCF(8,8) or CASSCF(8,8) wavefunction for p ‐benzyne . Although we agree with Lischka and co‐workers that a CAS(2,2) is too small for this system, a smaller CAS(6,6) space for ground‐state or vertical singlet–triplet gap computations (which is beyond the scope of this paper) seems to be sufficient on the basis of the FOD analysis.…”

“…A number of previous studies used a MCSCF(8,8) or CASSCF(8,8) wavefunction for p ‐benzyne . Although we agree with Lischka and co‐workers that a CAS(2,2) is too small for this system, a smaller CAS(6,6) space for ground‐state or vertical singlet–triplet gap computations (which is beyond the scope of this paper) seems to be sufficient on the basis of the FOD analysis. A twisted retinal structure, in which the dihedral angle around the double bond in position 11 was set to 90°, is our third example.…”

A comparison of neutral and charged species of one- and two-dimensional models of graphene nanoribbons using multireference theory

“…39 Size-extensivity contributions are included at the level of the multireference averaged quadratic coupled cluster (MR-AQCC) 40 approach, which has been successfully applied and found particularly useful for larger aromatic systems. 18,38,[41][42][43][44] Based on the MR-AQCC calculations, analysis of the polyradical nature of the acenes and periacenes is performed by two means: monitoring the evolution of the natural orbital (NO) occupation with increasing chain length and using the effectively unpaired density. The latter method was first proposed by Takatsuka et al 45 and was further developed by Staroverov and Davidson.…”

State‐specific multireference coupled‐cluster theory

“…Typically CASPT2 calculations are based on a rather large CAS to keep the perturbation correction sufficiently small, in case of the benzynes a CAS(8,8) 138. The same holds for MRCI calculations, which give, in particular for energies, more reliable results for larger active spaces as this diminishes the extent of the size‐consistency error 139. In contrast to that, MRCC calculations give rather accurate results for small active spaces, as shown in Table 1.…”