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The nature of chemical bonds on dynamic paths was investigated using the complete active space valence-bond (CASVB) method and the Born–Oppenheimer dynamics. To extract the chemical bond picture during reactions, a scheme to collect contributions from several VB (resonance) structures into a small numbers of indices was introduced. In this scheme, a tree diagram for the VB structures is constructed with the numbers of the ionic bonds treated as generation. A pair of VB structures is related to each other if one VB structure is transferred into the other by changing a covalent bond to an ionic bond. The former and latter VB structures are named parent and child structures, respectively. The weights of the bond pictures are computed as the sum of the CASVB occupation numbers running from the top generation to the bottom along the descent of the VB structures. Thus, a number of CASVB occupation numbers are collected into a small number of indices, and a clear bond picture may be obtained from the CASVB wave function. The scheme was applied to the hydrogen exchange reaction H2+F→H+HF and the Diels–Alder reaction C5H6(cyclopentadiene)+CH2=CH2(ethylene)→C7H10(norbornene). In both the reactions, the scheme gave a clear picture for the Born–Oppenheimer dynamics trajectories. The reconstruction of the bonds during reactions was well described by following the temporal changes in weight. [ABSTRACT FROM AUTHOR]

We have investigated the ground state and the two lowest excited states of the CeF molecule using four-component relativistic multiconfigurational quasidegenerate perturbation theory calculations, assuming the reduced frozen-core approximation. The ground state is found to be (4f1)(5d1)(6s1), with Ω=3.5, where Ω is the total electronic angular momentum around the molecular axis. The lowest excited state with Ω=4.5 is calculated to be 0.104 eV above the ground state and corresponds to the state experimentally found at 0.087 eV. The second lowest excited state is experimentally found at 0.186 eV above the ground state, with Ω=3.5 based on ligand field theory calculations. The corresponding state having Ω=3.5 is calculated to be 0.314 eV above the ground state. Around this state, we also have the state with Ω=4.5. The spectroscopic constants Re, ωe, and ν(1-0) calculated for the ground and first excited states are in almost perfect agreement with the experimental values. The characteristics of the CeF ground state are discussed, making comparison with the LaF+ and LaF molecules. We denote the d- and f-like polarization functions as d* and f*. The chemical bond of CeF is constructed via {Ce3.6+(5p6d*0.3f*0.1)F0.6-(2p5.6)}3+ formation, which causes the three valence electrons to be localized at Ce3.6+. [ABSTRACT FROM AUTHOR]

Excited states of three 7-aminocoumarin derivatives, coumarin 120 (7-amino-4-methylcoumarin), pyrrolyl coumarin 7-(1H-pyrrol-1-yl)-4-methyl-2H-1-benzopyran-2-one, and carbazole-coumarin hybrid 10H-4-methyl-2H-2-oxopyrano[5,6-b]carbazole, have been studied using B3LYP time-dependent density functional theory (TDDFT). The solvent effect has been taken into account using the polarizable continuum model. The spectra calculated using TDDFT agree well with the experimental absorption spectra. The electronic structures and the solvent effect for the low-lying singlet excited states have been investigated. The HOMO of the pyrrolyl coumarin is localized on the pyrrolyl ring, while the HOMO in the other 7-aminocoumarins is delocalized over the entire molecule. This leads to the weak fluorescence of the pyrrolyl coumarins found in experiments. The HOMO and next HOMO in carbazole-coumarin hybrids have similar orbital energy values, which is not the case in the other 7-aminocoumarin derivatives. This leads to the additional peaks found in the 30,000–40,000 cm-1 region of the observed absorption spectra, which are specific for carbazole-coumarin hybrids. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Multiconfigurational second-order quasidegenerate perturbation theory (MCQDPT) calculations were performed for the LaF+ molecule, with one LaF2+ and four LaF+ Dirac–Fock–Roothaan (DFR) spinor sets. The best spinor set was that of LaF2+, which gave the lowest total energies and also the best excitation energies for any state considered. The MCQDPT calculations with the cation and neutral molecular spinors were also performed for LaF. The MCQDPT with the cation spinors gave the lowest total energies for all states under consideration, and the calculated excitation energies compared best with experiment. We prefer the LaF+ spinor set to those of LaF. These calculations indicate that the DFR spinor set for the (n-1) electron system is adequate for treating the molecular electronic system having n electrons. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

A density functional theory method using partially fixed molecular orbitals (PFMOs) is presented. The PFMOs, which have some fixed molecular orbital coefficients and are non-orthogonal, are a generalization of the extreme localized orbitals (ELMOs) of Couty, Bayse, and Hall (1997) Theor Chem Acc 97:96. A non-orthogonal Kohn-Sham method with these PFMOs is derived, and is applied to molecular calculations on the ionization potential of pyridine, the energy difference between cis- and trans-butadiene, the reaction barrier height of the cyclobutene-cis-butadiene interconversion, and the potential energy curve of the hydrogen shift reaction of hydroxycarbene to formaldehyde. The PFMO Kohn-Sham method reproduces well the results of the full Kohn-Sham method without having a restriction on the molecular orbital coefficients. The difference is less than 0.1 eV in the ionization potential and about 0.1 kcal/mol in the barrier height and in the potential energy calculations. [ABSTRACT FROM AUTHOR]

Accurate quantum computational chemistry has evolved dramatically. The size of molecular systems, which can be studied accurately using molecular theory is increasing very rapidly. Theoretical chemistry has opened up a world of new possibilities. It can treat real systems with predictable accuracy. Computational chemistry is becoming an integral part of chemistry research. Theory can now make very significant contribution to chemistry. This review will focus on our recent developments in the theoretical and computational methodology for the study of molecular structure and molecular interactions. We are aiming at developing accurate molecular theory on systems containing hundreds of atoms. We continue our research in the following three directions: (i) development of new ab initio theory, particularly multireference-based perturbation theory, (ii) development of exchange and correlation functionals in density functional theory, and (iii) development of molecular theory including relativistic effects. We have enjoyed good progress in each of the above areas. We are very excited about our discoveries of new theory and new algorithms and would like to share this enthusiasm with readers. [ABSTRACT FROM AUTHOR]

[Display omitted] • The two-component transformation inclusive contraction (TIC) scheme is introduced. • The unitarized Douglas–Kroll (uDK) method is also introduced for TIC. • The TIC scheme produced highly accurate results for systems involving heavy atoms. • The uDK method was demonstrated to perform marginally better than conventional DK. The incorporation of a two-component transformation into the contraction coefficients of basis functions is suggested. Such a contraction, referred to as "two-component transformation inclusive contraction (TIC)", effectively eliminates the computational cost associated with the two-component transformation and can be readily implemented into conventional quantum chemistry programs. TIC was verified through numerical calculations using the second- and third-order unitarized Douglas–Kroll method, which is newly derived for the validation of TIC, and the infinite-order two-component method. The numerical validation results suggested that TIC could sufficiently reproduce the results of primitive basis sets for both atoms and molecules. [ABSTRACT FROM AUTHOR]

Abstract: The measurement of the electronic spectra of jet-cooled 4-chloro-7-azaindole dimer (4CAI2) revealed that the Cl substitution substantially suppresses the excited-state double-proton transfer (ESDPT). Quantum chemistry calculation at the CIS level suggested that the change in the intermolecular stretching mode, which promotes ESDPT, is important for the suppression of ESDPT. [Copyright &y& Elsevier]