A CASPT2 study of the valence and lowest Rydberg electronic states of benzene and phenol

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The two lowest bands, 1 B 2u and 1 B 1u , of the electronic spectrum of the benzene molecule have been studied theoretically using a new method to compute vibronic excitation energies and intensities. The complete active space ͑CAS͒ self-contained field ͑SCF͒ method ͑with six active -orbitals͒ was used to compute harmonic force field for the ground state and the 1 B 2u and 1 B 1u electronic states. A linear approximation has been used for the transition dipole as a function of the nuclear displacement coordinates. Derivatives of the transition dipole were computed using a variant of the CASSCF state interaction method. Multiconfigurational second-order perturbation theory ͑CASPT2͒ was used to obtain absolute excitation energies ͑12 active -orbitals͒. The results show that the approach works well. Vibrational progressions are well described in both bands and intensities, and energies are in agreement with experiment, in particular when CASPT2 derived geometries are used. One interesting result is that computed vertical energies fall about 0.1 eV on the high energy side of the band maximum.

Section: A Geometries and Energies Of The Ground And Excited Statesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

A theoretical study of the 1B2u and 1B1u vibronic bands in benzene

Bernhardsson

Forsberg

Malmqvist

et al. 2000

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“…Benzene is one of the most investigated molecules both theoretically [52][53][54][55][56][57][58][59][60] and experimentally. [60][61][62][63][64][65][66][67][68][69][70][71][72][73][79][80][81][82] However, there are still many unsolved problems in the benzene spectrum.…”

Section: Introductionmentioning

confidence: 99%

Large-scale calculations of excitation energies in coupled cluster theory: The singlet excited states of benzene

Christiansen

Koch

Halkier

et al. 1996

196

134

Algorithms for calculating singlet excitation energies in the coupled cluster singles and doubles ͑CCSD͒ model are discussed and an implementation of an atomic-integral direct algorithm is presented. Each excitation energy is calculated at a cost comparable to that of the CCSD ground-state energy. Singlet excitation energies are calculated for benzene using up to 432 basis functions. Basis-set effects of the order of 0.2 eV are observed when the basis is increased from augmented polarized valence double-zeta ͑aug-cc-pVDZ͒ to augmented polarized valence triple-zeta ͑aug-cc-pVTZ͒ quality. The correlation problem is examined by performing calculations in the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3, as well as by using the CCSDR͑3͒ perturbative triples corrections. The effect of triple excitations are less than 0.2 eV for all excitations except for the 2 1 E 2g state. The calculated excitation energies are compared with experiment and other theoretical results.

“…49,69 The two lowest triplet states 1 3 B u and 1 3 A g estimated at the MS-RASPT2 level for both butadiene and hexatriene are close to those obtained by Silva et al 48 and experimental data. 58 As shown in the past, 47 benzene also has valenceRydberg mixing like the polyenes discussed before. In this aromatic molecule, the erratic valence state is 1 1 E 1u but MS-RASPT2 solved it properly as shown previously.…”

Section: B Excitation Energiesmentioning

confidence: 66%

“…The active space used is comprised by four σ orbitals placed in RAS1, six orbitals in RAS2 (the π and its correlating π * orbital plus two more of b 2g symmetry, one of them being the 3d xz Rydberg orbital, and the two (σ , σ *) molecular orbitals (MOs) of the carbon-carbon bond), and a total of eight orbitals in RAS3 (corresponding to the antibonding counterparts of the four σ MOs plus extra correlating orbitals in order to minimize the appearance of intruder states resulting from the use of an extended basis set.). 47,48 Twelve electrons are active and up to two holes/particles are allowed in RAS1/RAS3. Using the notation from a recent benchmark calibration, 40 the level of calculation of the present work is labelled as MS-RASPT2(12,2,2;4,6,8)(SD), where within parentheses the number of active electrons (12), number of holes (2) and particles (2), as well as the number of active orbitals 4, 6, and 8 for RAS1, RAS2, and RAS3, respectively, are specified.…”

Section: Computation Of Potential Energy Curves For Stretched Etmentioning

confidence: 99%

A distance-dependent parameterization of the extended Hubbard model for conjugated and aromatic hydrocarbons derived from stretched ethene

Schmalz

Serrano‐Andrés

Sauri

et al. 2011

The Hubbard model, which is widely used in physics but is mostly unfamiliar to chemists, provides an attractive yet simple model for chemistry beyond the self consistent field molecular orbital approximation. The Hubbard model adds an effective electron-electron repulsion when two electrons occupy the same atomic orbital to the familiar Hückel Hamiltonian. Thus it breaks the degeneracy between excited singlet and triplet states and allows an explicit treatment of electron correlation. We show how to evaluate the parameters of the model from high-level ab initio calculations on twoatom fragments and then to transfer the parameters to large molecules and polymers where accurate ab initio calculations are difficult or impossible. The recently developed MS-RASPT2 method is used to generate accurate potential energy curves for ethene as a function of carbon-carbon bond length, which are used to parameterize the model for conjugated hydrocarbons. Test applications to several conjugated/aromatic molecules show that even though the model is very simple, it is capable of reasonably accurate predictions for bond lengths, and predicts molecular excitation energies in reasonable agreement with those from the MS-RASPT2 method.