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The key role of the molecular orbitals in describing electron transfer processes is put in evidence for the intervalence charge transfer (IVCT) of a synthetic nonheme binuclear mixed-valence Fe3+/Fe2+ compound. The electronic reorganization induced by the IVCT can be quantified by controlling the adaptation of the molecular orbitals to the charge transfer process. We evaluate the transition energy and its polarization effects on the molecular orbitals by means of ab initio calculations. The resulting energetic profile of the IVCT shows strong similarities to the Marcus' model, suggesting a response behaviour of the ensemble of electrons analogue to that of the solvent. We quantify the extent of the electronic reorganization induced by the IVCT process to be 11.74 eV, a very large effect that induces the crossing of states reducing the total energy of the transfer to 0.89 eV. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The n-electron valence state perturbation theory makes use ofzero-order wave functions whose energies are endowed with a directphysical interest, describing various processes occurring in the activespace (removal/addition of one or two electrons, electronic excitations).It is shown that the zero-order energies related to the process ofremoval of an electron from the active space provide a reasonableand cheap approximation to the vertical ionization potentials. Thezero-order energies referring to the process of an electronic excitationwithin the active space can also provide a first approximation toelectronic transition energies, provided that a careful choice ofthe active molecular orbitals is performed. Test calculations havebeen carried out on the molecules N2and H2CO. [ABSTRACT FROM AUTHOR]

The electronic structure of the XO and XO+ (X = I, At) species, as well that of a AtO+–H2O complex have been investigated using relativistic wave-function theory and density functional theory (DFT)-based approximations (DFAs). The n-electron valence state perturbation method with the perturbative inclusion of spin–orbit coupling including spin–orbit polarization effects (SO-NEVPT2) was shown to yield transition energies within 0.1 eV of the reference four-component intermediate Fock-space coupled cluster (DC-IHFSCCSD) method at a significantly lower computational cost and can therefore be used as a benchmark to more approximate approaches in the case of larger molecular systems. These wavefunction calculations indicate that the ground state for the AtO+ and AtO+–H2O systems is the Ω = 0+ component of the 3Σ− LS state, which is quite well separated (by ≃0.5 eV) from the Ω = 1 components of the same state and from the Ω = 2 state related to the 1Δ LS state (by ≃1 eV). Time-dependent DFT calculations, on the other hand, place the Ω = 1 below the Ω = 0+ component with the spurious stabilization of the former increasing as one increases the amount of Hartree–Fock exchange in the DFAs, while those employing the Tamm–Dancoff approximation and DFAs not including Hartree–Fock exchange yield transition energies in good agreement with SO-NEVPT2 or DC-IHFSCCSD for the lower-lying states. These results indicate the ingredients necessary for devising a DFA-based computational protocol applicable to the study of the properties of large AtO+ clusters so that it may (at least) qualitatively reproduce reliable reference (SO-NEVPT2) calculations. [ABSTRACT FROM AUTHOR]

Code interoperability and the search for domain-specific standard data formats represent critical issues in many areas of computational science. The advent of novel computing infrastructures such as computational grids and clouds make these issues even more urgent. The design and implementation of a common data format for quantum chemistry (QC) and quantum dynamics (QD) computer programs is discussed with reference to the research performed in the course of two Collaboration in Science and Technology Actions. The specific data models adopted, Q5Cost and D5Cost, are shown to work for a number of interoperating codes, regardless of the type and amount of information (small or large datasets) to be exchanged. The codes are either interfaced directly, or transfer data by means of wrappers; both types of data exchange are supported by the Q5/D5Cost library. Further, the exchange of data between QC and QD codes is addressed. As a proof of concept, the H + H2 reaction is discussed. The proposed scheme is shown to provide an excellent basis for cooperative code development, even across domain boundaries. Moreover, the scheme presented is found to be useful also as a production tool in the grid distributed computing environment. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In an endeavor to study how a polydentate nitrogen donor ligand affects the magnetic properties of copper(ii) methoxybenzoates, three novel complexes of copper(ii) were investigated. [Cu2(H2tea)2(o-methoxybenzoate)2], [Cu2(H2tea)2(m-methoxybenzoate)2]·2H2O and [Cu2(H2tea)2(p-methoxybenzoate)2]·2H2O (where H2tea = mono-deprotonated triethanolamine) were synthesized by addition of triethanolamine (H3tea) to the hydrated Cu(o-,m-,p-methoxybenzoates)2. The newly synthesized complexes have been characterized by elemental analyses, spectroscopic techniques (electronic and FT-IR), magnetic moment determination, molar conductance studies, TGA, and single crystal X-ray determination. The experimental characterization was integrated with the ab initio theoretical determination of the magnetic coupling constant value and with the analysis of the correlation between this value and the relevant geometrical parameters. Variable temperature solid state magnetization measurements and ab initio calculations indicate a remarkable ferromagnetic coupling of the unpaired electrons centered on the two Cu atoms for the m- and p-methoxybenzoate complexes (J = 100.9 cm−1), while a non-negligible antiferromagnetic coupling is found for the third complex (J = −83.1 cm−1). This differential behaviour can be rationalized on the basis of the out-of-plane displacement (τ) of the alkoxo group with respect to the molecular Cu2O2 plane. Large τ values prevent an efficient overlap between the O 2p and the magnetic Cu 3dx2−y2, favouring a ferromagnetic coupling between the Cu sites. [ABSTRACT FROM AUTHOR]

The dependence on the temperature of the population of the ith state, Pi, in the Boltzmann distribution is analyzed by studying its derivative with respect to the temperature, T. A simple expression is found, involving Pi, the energy of the state, Ei, and the average energy, (E). This relation is completely general (it has the same form in all the thermodynamic ensembles), and it has a relevant didactic content, given that it predicts the qualitative variation of Pi with T even in complex systems. The derivation of this relation, the discussion of its properties, and its application to simple problems is appropriate for a statistical thermodynamics course in the chemistry curriculum. [ABSTRACT FROM AUTHOR]

A computational strategy is presented to describe excited states, involving the transfer of an electron from one metallic site to a neighboring metal center, the so-called metal-to-metal charge-transfer (MMCT) states. An accurate ab initio treatment of these states in transition metal compounds is intrinsically difficult for both time-dependent density functional and wave function-based methods. The rather large dependence of the MMCT energies on the applied functional makes difficult to extract reliable estimates from density functional theory, while the standard multiconfigurational approach (complete active space SCF + second-order perturbation theory) leads to severe intruder state problems and unrealistic, negative energies. The analysis of the failure of the multiconfigurational approach shows that the state-average orbitals are biased toward the ground state and strongly deficient to describe the MMCT state. We propose a method to improve the orbitals by gradually approaching as much as possible the state-specific description of the MMCT state in the reference wave function for the second-order perturbation treatment of the dynamic electron correlation. [ABSTRACT FROM AUTHOR]

The [image omitted] 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 atoms. At all levels of calculation the [image omitted] 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 [image omitted] 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 calculation published thus far. [ABSTRACT FROM AUTHOR]

The magnetic coupling in organic biradicals has been analyzed by means of ab initio wave function-based methods. Attention is focused on the coupling between the spin moments localized on the NO-groups in meta and para phenylene-bridged nitroxides, and bis(nitronyl) nitroxide and bis(imino) nitroxide biradicals. The leading mechanisms governing the coupling have been isolated by means of class-partitioned CI calculations. It was found that the mechanisms of the coupling in the para and meta phenylene-bridged nitroxides are similar to that found in transition metal complexes, while for the other biradicals the dominance of other mechanisms (like the spin polarization) imposes restrictions on the computational strategy to be followed to best estimate the coupling. [ABSTRACT FROM AUTHOR]

In the last two decades, difference dedicated configuration interaction (DDCI) calculations have been outlined as a rather accurate and a successful approach for the evaluation of magnetic coupling constants J. The DDCI space contains all single and double semi- active excitations on the top of the complete active space, which is only composed of the magnetic orbitals. The main drawback of the procedure is its computational cost, in particular for systems involving several metallic centres with S > ½. To overcome this constraint, an alternative strategy has been proposed in the past which consists in restricting the CI expansion to single excitations on the top of an extended active space, involving a few doubly occupied and in some cases virtual orbitals of ligand character. This strategy reduces considerably the size of the CI matrices, and then makes possible to deal with larger and more complex systems. However, the so obtained J values are overestimated compared to the corresponding DDCI and to the experimental values. We analyze here the origin of this behavior in the light of a recent paper where the role of ligand-to-metal charge transfer excitations on the quantitative description of J has been re-examined. [ABSTRACT FROM AUTHOR]

This paper addresses the study of the electronic spectrum of the s-tetrazine molecule in the ab initio frame making use of the multireference n-electron Valence State Perturbation Theory (NEVPT2). The theoretical description of the excited states of this molecule is complex, because of the different computational requirements of the low-lying excited states which must be treated on an equal footing. More than forty electronic excited states of various nature ( n → π*, nn → π*π*, π → π*, nπ → π*π*, and Rydberg) are considered here. Various active spaces are used to reach a good quality zero order description, needed to avoid unrealistic results in the perturbation treatment. The quasi-degenerate perturbation theory must be used in most of the cases, given the presence of a marked mixing among various states. The results here presented have been used to add new information to the interpretation of the experimental spectra. While in many cases previous assignments of the experimental features are confirmed, in various cases they are questioned and new assignments are proposed. With the new assignments, a good agreement is found between experiments and NEVPT2. The comparison with other high level ab initio methods shows that NEVPT2 performs well, being in general in close agreement with Extended-STEOM-CCSD and CC3, while the agreement with CASPT2 and GVVPT2 is less satisfactory (the CASPT2 excitation energies being lower than the NEVPT2 ones by ≃0.4 eV). Finally, comparing NEVPT2 with TD-DFT, a reasonable accordance with the values obtained with the PBE0 functional is observed, while the agreement with those computed with the HCTH functional is lower. [ABSTRACT FROM AUTHOR]

The article focuses on the use of the orthogonal decomposition of the ground state wavefunction for the fundamental bifurcation in the valence-bond (VB) theory method. According to the authors, the bond electron pairs in the model is in agreement with Lewis' basic idea which built from two atomic orbitals. They noted that the orbitals are either the spectroscopic valence orbitals of s, p or d character or hybrid atomic orbitals directed along the bond axis. They concluded that non-orthogonal pure atomic orbitals is closer to intuition while neutral VB configuration brings a major contribution to the binding energy.

The vertical electronic spectrum of the thiophene molecule is investigated by means of second and third order multireference perturbation theory (NEVPT). Single-state and quasi-degenerate NEVPT calculations of more than 25 singlet excited states have been performed. The study is addressed to the theoretical characterization of the four lowest-energy $${\pi\to\pi^*}$$ valence states, as well as the 3 s, 3 p and 3 d Rydberg states. In addition, the excitation energies of two $${\pi\to\sigma^*}$$ and $${n\to\pi^*}$$ valence states are also reported. For almost all the excited states, coupled cluster calculations (CCSD and CCSDR(3)) have been also carried out, using the same geometry and basis set used for the NEVPT ones, in order to make the comparison between the results of the two methods meaningful. A remarkable accordance between the NEVPT and CC excitation energies is found. The present results, over all, confirm the experimental assignments but, above all, represent an important contribution to the assignments of some low-energy $${\pi\to\sigma^*}$$ states, valence and Rydberg, for which a firm interpretation is not available in the literature. [ABSTRACT FROM AUTHOR]

The n-electron valence state perturbation theory (NEVPT) is a form of multireference perturbation theory which is based on a zero order reference wavefunction of CAS-CI type (complete active space configuration interaction) and which is characterized by the utilization of correction functions (zero order wavefunctions external to the CAS) of multireference nature, obtained through the diagonalization of a suitable two-electron model Hamiltonian (Dyall’s Hamiltonian) in some well defined determinant spaces. A review of the NEVPT approach is presented, starting from the original second order state-specific formulation, going through the quasidegenerate multi-state extension and arriving at the recent implementations of the third order in the energy and of the internally contracted configuration interaction. The chief properties of NEVPT—size consistence and absence of intruder states—are analyzed. Finally, an application concerning the calculation of the vertical spectrum of the biologically important free base porphin molecule, is presented. [ABSTRACT FROM AUTHOR]

This paper first comments on the surprisingly poor status that Quantum Chemistry has offered to the fantastic intuition of Lewis concerning the distribution of the electrons in the molecule. Then, it advocates in favor of a hierarchical description of the molecular wave-function, distinguishing the physics taking place in the valence space (in the bond and between the bonds), and the dynamical correlation effects. It is argued that the clearest pictures of the valence electronic population combine two localized views, namely the bond (and lone pair) Molecular Orbitals and the Valence Bond decomposition of the wave-function, preferably in the orthogonal version directly accessible from the complete active space self consistent field method. Such a reading of the wave function enables one to understand the work of the nondynamical correlation as an enhancement of the weight of the low-energy VB components, i.e. as a better compromise between the electronic delocalization and the energetic preferences of the atoms. It is suggested that regarding the bond building, the leading dynamical correlation effect may be the dynamical polarization phenomenon. It is shown that most correlation effects do not destroy the bond electron pairs and remain compatible with Lewis' vision. A certain number of free epistemological considerations have been introduced in the development of the argument. © 2006 Wiley Periodicals, Inc. J Comput Chem 2007 [ABSTRACT FROM AUTHOR]

Abstract: The problem of the development of salt concentration profiles in a solar pond is investigated, the thermodiffusion contribution is also taken into account, using a one-dimensional mathematical model and a finite-difference approach. A novel scheme of heat extraction from the solar pond is presented, along with preliminary two-dimensional computational fluid dynamics (CFD) simulations. [Copyright &y& Elsevier]

In this paper a simple approximate approach for the study of quasi-degenerate systems is presented in the frame of multireference perturbation theory. The formulation can be considered as an approximation of the quasi-degenerate perturbation theory (QDPT) with the simplification that only the state specific (diagonal) perturbation corrections to the energy have to be computed. The new approach is discussed and compared with previous QDPT formulations using the weakly avoided crossing model (for which new properties are here presented) and applied to the case of the neutral/ionic energy crossing in the LiF molecule. [ABSTRACT FROM AUTHOR]

Abstract: The vertical electronic spectrum of furan is investigated by second and third-order multireference perturbation theory (NEVPT). The excitation energies of the three lowest-energy valence states, as well as the 3l Rydberg states are reported. The effects of the size of the active space and the valence–Rydberg mixing are discussed. The application of the quasi-degenerate NEVPT approach has proved to be necessary in order to handle the consistent valence–Rydberg interactions occurring for the two valence states. For the three valence states and the low-lying Rydberg states, the computed excitation energies agree with those computed in the more recent high-level theoretical studies. [Copyright &y& Elsevier]

Abstract: The vertical electronic spectrum of pyrrole is investigated by means of second and third order n-electron valence state perturbation theory. The three , and valence states, as well as the 3s, 3p and 3d π- and σ-type Rydberg states, are considered. Particular attention is paid to the description of the valence states, where different active spaces of increasing size are used to improve the zero order wave function. For the Rydberg states and the covalent valence state , the perturbative results show a coherent trend and are in accordance with those of the previous high-level studies. For the two ionic valence states ( and ), rather large active spaces are required to get satisfactory results. [Copyright &y& Elsevier]

A recently proposed a priori localization technique is used to exploit the possibility to reduce the number of active orbitals in a Complete Active Space Self Consistent Field calculation. The work relies on the fact that the new approach allows a strict control on the nature of the active orbitals and therefore makes it possible to include in the active space only the relevant orbitals. The idea is tested on the calculation of the energy barrier for rigid rotation of linear polyenes. In order to obtain a relevant set of data, a number of possible rotations around double bonds have been considered in the ethylene, butadiene, hexatriene, octatetraene, decapentaene, dodecahexaene molecules. The possibility to reduce the dimension of the active space has been investigated, considering for each possible rotation different active spaces ranging from the minimal dimension of 2 electrons in 2?p orbitals to the p-complete space. The results show that the rigid isomerization in the polyene molecules can be described with a negligible loss in accuracy with active spaces no larger than ten orbitals and ten electrons. In the special case of the rotation around the terminal double bond, the space can be further reduced to six orbitals and six electrons with a large decrease of the computational cost. An interesting summation rule has been found and verified for the stabilization of the energy barriers as a function of the dimension of the conjugated lateral chains and of the dimension of the active space. [ABSTRACT FROM AUTHOR]

A short summary of the n-electron valence state perturbation theory (NEVPT) is provided, and the defects inherent in cases of quasi-degeneration are underscored. A new version of the theory, called QD-NEVPT, based on quasi-degenerate perturbation theory, is presented. The new theory introduces an effective interaction among the zero-order wave functions and overcomes the difficulties of state-specific NEVPT. An application concerning the interaction among the π → π* and Rydberg singlets of formaldehyde is presented. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]