A convenient decontraction procedure of internally contracted state-specific multireference algorithms

Internally contracted state-specific multireference MR algorithms, either perturbative such as CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR coupled cluster, are computationally efficient but they may suffer from the internal contraction of the wave function in the reference space. The use of a low dimensional multistate model space only offers limited flexibility and is not always practicable. The present paper suggests a convenient state-specific procedure to decontract the reference part of the wave function from a series of state-specific calculations using slightly perturbed zero-order wave functions. The method provides an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the excited roots of which are shown to be relevant. The orthogonal valence bond functions can be considered quasidiabatic states and the effective valence Hamiltonian gives therefore the quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of the method is illustrated in two case problems where the dynamical correlation plays a crucial role, namely, the LiF neutral/ionic avoided crossing and the F2 ground state wave functio

Photoionization of furan from the ground and excited electronic states

Here we present a comparative computational study of the photoionization of furan from the ground and the two lowest-lying excited electronic states. The study aims to assess the quality of the computational methods currently employed for treating bound and continuum states in photoionization. For the ionization from the ground electronic state, we show that the Dyson orbital approach combined with an accurate solution of the continuum one particle wave functions in a multicenter B-spline basis, at the density functional theory (DFT) level, provides cross sections and asymmetry parameters in excellent agreement with experimental data. On the contrary, when the Dyson orbitals approach is combined with the Coulomb and orthogonalized Coulomb treatments of the continuum, the results are qualitatively different. In excited electronic states, three electronic structure methods, TDDFT, ADC(2), and CASSCF, have been used for the computation of the Dyson orbitals, while the continuum was treated at the B-spline/DFT level. We show that photoionization observables are sensitive probes of the nature of the excited states as well as of the quality of excited state wave functions. This paves the way for applications in more complex situations such as time resolved photoionization spectroscopy

Many-body multireference Moeller-Plesset and Epstein-Nesbet perturbation theory: fast evaluation of second-order energy contributions

Multireference perturbation theory is examined in connection with the two partitions in the Møller - Plesset and Epstein - Nesbet schemes. The implementation of an efficient diagrammatic technique is described and two examples of application (diazene and the Cr2 molecule), involving large variational spaces, are provide

Nonempirical investigations on the azomethine group. 1. The effect of the conjugation with a phenyl ring. A comparison of the electronic structure of the ground and lowest excited states in benzaldimine and N- phenylformaldimine

The possibility of differential effects due to conjugation with a phenyl ring when linked either to the C or N end of the azomethine group is investigated by means of ab initio calculations on the ground and lowest excited states of benzaldimine, N-phenylformaldimine, and the parent compound methylenimine. For the description of the excited states, the electron-hole potential method by Morokuma and Iwata has been adopted. Population analysis shows that the ability of the azomethine group to accept or to donate electrons in a given state is practically the same in molecules 1 and 2. A parallel analysis of the electrostatic molecular potential confirms the results of the papulation analysis and in addition shows differences in reactivity between the two molecules in a given state and among the various states in each molecule

A Multireference Perturbation Theory Study on the Fe₂ molecule: in quest of the ground state.

International audienceThe 9Σ-g and 7Δu states of the iron dimer, which according to a long-standing debate are the most probable candidates for the assignment of the ground state, have been investigated within the frame of multireference perturbation theory (PC-NEVPT2 and SC-NEVPT3) up to the third order in the energy. At short internuclear distances the 7Δu state originates from the interaction of a ground-state 5D Fe atom with an excited-state 5F, whereas at large distances it results from the Van der Waals interaction of two 5D Fe atoms. At all levels of calculation the 9Σ-g term appears to have lower energy than the short-range 7Δu state and can thus be assigned as the ground state of the iron dimer. The spectroscopic constants of the 9Σ-g state (Re=2.164 Å, ωe=302.6 cm-1 at the SC-NEVPT3 level of approximation) are in good accordance with the other high level ab initio calculations published thus far

Revival of an old structure problem: Trithiapentalene - real or time-averaged C2v symmetry?

The structures of 1, 6,6a\Lambda4-trithiapentalene la and the related compounds 1, 6-dioxa-6a\Lambda44-thiapentalenel b and I,6-diaza-6ah4-thiapentaleneI C were examined on the basis of ab initio MO theory employing the 3-21G(*) and 6-31G* basis sets. In case of the trithia and dioxathia molecules, the bridged C, structures appear as most stable arrangements when the correlation energy is considered, whereas the corresponding open C, forms are preferred at the SCF level. The same conclusions can be drawn for the 1,6-disubstituted diazathiapentalenes. Contrary to this, structure 4 with an aromatic isothiazole ring is the most stable form for the unsubstituted compounds

Conformation of 1,4-dihydropyridine - planar or boat-like?

The geometry of the 1,4-dihydropyridine molecule was completely optimized employing three different ab initio basis sets (6–31 G*, 4–31 G, STO—3G). The most reliable 6–31G* basis set provides a very flat boat conformation which may easily undergo defolding to a planar ring arrangement. This result is discussed with respect to enzymatic redox cofactors and the pharmacological activity of dihydropyridine calcium antagonists

Comments on the diabatic representation

The concept of diabatic states, which has proved to be fruitful in a variety of molecular studies, is here analysed. The one-dimensional case is examined in the light of the asymptotic behaviour