Biomedical subjects

Christof Hättig

Publications and source records attributed to Christof Hättig.

17 recordsLinked to original sources

PubMed2006

Intramolecular charge-transfer mechanism in quinolidines: the role of the amino twist angle.

Quantum-chemical calculations with the approximate coupled-cluster singles-and-doubles model CC2 have been carried out for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6). For this molecule dual fluorescence was experimentally observed, raising the discussion about the importance of the amino twist angle for this process. The calculations suggest that both the ground state and the normal fluorescent state are significantly twisted by 30 degrees -40 degrees and that the molecule is flexible enough to move into an even stronger twisted conformation (60 degrees -70 degrees ) in its intramolecular charge-transfer (ICT) state which is responsible for the anomalous fluorescence band. Such a conformation both minimizes the total energy in the S1 state and maximizes the dipole moment. The barrier from the normal fluorescent state to the ICT state region is very small. Comparison to the situation in the 1-methyl-derivative NMC6 suggests that a large alkyl substituent makes the preferably planar normal fluorescent state energetically unfavorable compared to the ICT state and thus promotes the occurrence of dual fluorescence.

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PubMed2006

Femtosecond pump/probe photoelectron spectroscopy of isolated C60 negative ions.

We have measured pump/probe photoelectron spectra of mass-selected, near room temperature C60- in the gas phase. The lifetime of the vibrationally excited B- (2Eg) state at a calculated energy of 1.26 eV was found to be tau = 2.2+/-0.2 ps. The dominant decay process corresponds to intramolecular radiationless transitions into ground state C60-. This is in contrast to C60 for which pumping at the absorption onset (1.95 eV) leads to predominantly intersystem crossing.

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PubMed2006

Extensions of r12 corrections to CC2-R12 for excited states.

As known since about two decades, R12 methods, which include terms linear in the interelectronic distance r(12) in the wave function, improve substantially the basis set convergence of the ground state correlation energy. In a previous study, however, it was found that the same approach does not give a similar systematic improvement if applied to excited states in the framework of coupled cluster response theory. In the present work, we examine the reason for this behavior and show that the inclusion of additional orbitals in the construction of the r(12) pair functions leads to an enhanced basis set convergence (and thus a balanced description) also for the excited states.

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PubMed2006

Microwave and theoretical investigation of the internal rotation in m-cresol.

The microwave spectrum of m-cresol (3-methylphenol) has been investigated using a molecular beam Fourier transform microwave spectrometer in the frequency range from 3 to 26.5 GHz. The rotation of the hydroxy group into two different unequal energetic minima leads to different spectra for the syn- and anticonformers. Because of a high potential barrier both conformers can be analyzed independently. The methyl group is undergoing an almost free internal rotation which is only hindered by small barriers and splits the vibrational ground state in two states of internal rotation denoted as A and E species. The spacing between the species is found to be up to 10 GHz. The potential for the internal rotation can be determined from the spectra and analyzed in terms of the Fourier components V3 and V6. For syn-m-cresol these parameters were determined as V3=673(3) GHz and V6=-335(24) GHz and for anti-m-cresol V3=95(5) GHz and V6=-416(46) GHz. The barriers to internal rotation were furthermore calculated with second-order Moller-Plesset perturbation theory and second-order coupled-cluster singles- and-doubles model (CC2) in the electronic ground state and with CC2 in the first excited state. The CC2 method is found to be an appropriate method to calculate potential barriers in electronic excited states of such compounds.

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PubMed2006

Photophysics of organic photostabilizers. Ab initio study of the excited-state deactivation mechanisms of 2-(2'-hydroxyphenyl)benzotriazole.

Excited-state reaction paths and the corresponding energy profiles of 2-(2'-hydroxyphenyl)benzotriazole (TIN-H) have been determined with the CC2 (simplified singles-and-doubles coupled-cluster) ab initio method. Hydrogen transfer along the intramolecular hydrogen bond, torsion of the aromatic rings and pyramidization of the central nitrogen atom are identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer is found to be unstable with respect to torsion. The latter mode, together with a moderate pyramidization of the central nitrogen atom, provides barrierless access to a S(1)-S(0) conical intersection. Only the pi-type orbitals of the aromatic rings are involved in the open-shell structures. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the aromatic rings, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the enol-type minimum of the S(0) potential-energy surface, thus closing the photocycle. This photophysical pathway accounts for the remarkable photostability of the molecule.

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PubMed2006

High-order correlation effects on dynamic hyperpolarizabilities and their geometric derivatives: a comparison with density functional results.

Second harmonic generation hyperpolarizabilities and their geometric derivatives have been calculated for HF, HCl, CO, and LiF, using the coupled cluster hierarchies, CCS, CC2, CCSD, CC3, and large correlation-consistent basis sets. The full configuration interaction results have been used to test the accuracy of the coupled cluster results. The CCS and CC2 methods do not improve on the Hartree-Fock results while CCSD is robust and gives significant improvements compared to CCS and CC2. The effects of triples in CC3 are in some cases substantial. Higher order correlation effects are significant for LiF. Including core-valence correlation effects is required only if high accuracy is desired. The coupled cluster results have been used as benchmarks for the results obtained by means of density functional theory using various exchange-correlation functionals. For the hyperpolarizability B3LYP was found to perform best, i.e., to give the results closest to the CC3 ones, while for the geometric derivatives none of the considered functionals was able to give a consistent description for all the considered molecules.

Journal Article↗

PubMed2006

Benchmarking two-photon absorption with CC3 quadratic response theory, and comparison with density-functional response theory.

We present a detailed study of the effects of electron correlation on two-photon absorption calculated by coupled cluster quadratic response theory. The hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3 has been used to investigate the effects of electron correlation on the two-photon absorption cross sections of formaldehyde (CH2O), diacetylene (C4H2), and water (H2O). In particular, the effects of triple excitations on two-photon transition cross sections are determined for the first time. In addition, we present a detailed comparison of the coupled cluster results with those obtained from Hartree-Fock and density-functional response theories. We have investigated the local-density approximation, the pure Becke-Lee-Yang-Parr (BLYP) functional, the hybrid Becke-3-parameter-Lee-Yang-Parr (B3LYP), and the Coulomb-attenuated B3LYP (CAM-B3LYP) functionals. Our results show that the CAM-B3LYP functional, when used in conjuction with a one-particle basis-set containing diffuse functions, has much promise; however, care must still be exercised for diffuse Rydberg-type states.

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PubMed2006

Distributed memory parallel implementation of energies and gradients for second-order Møller-Plesset perturbation theory with the resolution-of-the-identity approximation.

We present a parallel implementation of second-order Møller-Plesset perturbation theory with the resolution-of-the-identity approximation (RI-MP2). The implementation is based on a recent improved sequential implementation of RI-MP2 within the Turbomole program package and employs the message passing interface (MPI) standard for communication between distributed memory nodes. The parallel implementation extends the applicability of canonical MP2 to considerably larger systems. Examples are presented for full geometry optimizations with up to 60 atoms and 3300 basis functions and MP2 energy calculations with more than 200 atoms and 7000 basis functions.

Journal Article↗

PubMed2006

Coupled-cluster response theory with linear-r12 corrections: the CC2-R12 model for excitation energies.

Coupled-cluster response theory for vertical excitation energies within the second-order approximate coupled-cluster singles-and-doubles model CC2, including linear-r(12) corrections, is derived and implemented for Ansatze 1 and 2 of R12 theory. An orthonormal auxiliary basis set is used for the resolution-of-the-identity approximation in order to calculate the three- and four-electron integrals needed in R12 theory. The basis set convergence is investigated for a selected set of atoms and small molecules and it is found that in many cases the convergence is not improved. An analysis of the different contributions to excitation energies shows that the present scheme for the construction of the R12 pair functions leads in response theory to an unbalanced description of ground- and excited-state wave functions and needs to be generalized to carry the high accuracy of R12 methods over to response theory.

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PubMed2005

The accuracy of ab initio molecular geometries for systems containing second-row atoms.

The performance of the standard hierarchy of ab initio models--that is, Hartree-Fock theory, second-order Moller-Plesset theory, coupled-cluster singles-and-doubles theory, and coupled-cluster singles-doubles-approximate-triples theory--in combination with correlation-consistent basis sets is investigated for equilibrium geometries of molecules containing second-row elements. From an analysis on a collection of 31 molecules (yielding statistical samples of 41 bond distances and 13 bond angles), the statistical errors (mean deviation, mean absolute deviation, standard deviation, and maximum absolute deviation) are established at each level of theory. The importance of core correlation is examined by comparing calculations in the frozen-core approximation with calculations where all electrons are correlated.

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PubMed2005

Frequency-dependent hyperpolarizabilities of the Ne, Ar, and Kr atoms using the approximate coupled cluster triples model CC3.

The frequency-dependent electric field-induced second harmonic generation (ESHG) second hyperpolarizabilities gamma of neon, argon, and krypton are calculated using the approximate coupled cluster triples model CC3. Systematic basis set investigations are carried out to establish basis set limits, and scalar relativistic effects are accounted for by direct perturbation theory. To estimate higher-order correlation effects, full configuration-interaction results are used to benchmark the accuracy of CC3. The best theoretical estimates obtained thereby for the static second hyperpolarizabilities gamma(0) are 107.4, 1159, and 2589 a.u. for neon, argon, and krypton, respectively. These values as well as the results for the dispersion curve of the parallel component gamma( parallel) agree well with the latest experimental values from electric field-induced second harmonic generation. In addition, the dispersion of the perpendicular component gamma( perpendicular) and the hyperpolarizability ratios gamma( parallel)gamma( perpendicular) has been studied for the first time on a consistently correlated ab initio level. The analysis of the results indicates that, in particular for neon and krypton, the presently available experimental values are flawed.

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PubMed2005

Excited-state intramolecular proton transfer: a survey of TDDFT and RI-CC2 excited-state potential energy surfaces.

TDDFT and RI-CC2 calculations have been performed on the excited-state intramolecular proton transfer in malonaldehyde, o-hydroxybenzaldehyde, salicylic acid, 7-hydroxy-1-indanone, and 2-(2'-hydroxyphenyl)benzothiazole. Vertical and adiabatic excitation energies have been computed for the npi and pipi states. Overall, we have found that both RI-CC2 and TDDFT methods are good candidates for the description of ESIPT potential energy surfaces. Proton transfer (PT) curves have been computed for both excited states. An essentially barrierless and very shallow energy profile has been found for the pi pi* state. For the n pi* state the keto minimum is more pronounced than for the pi pi* state and, depending on the case, energy barriers ranging from values <0.1 eV up to 0.5 eV were found. From the computed PT curves we conclude that extended crossing regions between the two excited states will occur.

Computer Simulation↗

PubMed2005

Coupled-cluster theory with simplified linear-r(12) corrections: the CCSD(R12) model.

A simplified singles-and-doubles linear-r(12) corrected coupled-cluster model, denoted CCSD(R12), is proposed and compared with the complete singles-and-doubles linear-r(12) coupled-cluster method CCSD-R12. An orthonormal auxiliary basis set is used for the resolution-of-the-identity approximation to calculate three-electron integrals needed in the linear-r(12) Ansatz. Basis-set convergence is investigated for a selected set of atoms and small molecules. In a large basis, the CCSD(R12) model provides an excellent approximation to the full linear-r(12) energy contribution, whereas the magnitude of this contribution is significantly overestimated at the level of second-order perturbation theory.

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PubMed2004

The Cotton-Mouton effect of neon and argon: a benchmark study using highly correlated coupled cluster wave functions.

The Cotton-Mouton effect (magnetic field induced linear birefringence) has been studied for neon and argon using state-of-the-art coupled cluster techniques. The coupled cluster singles, doubles and triples (CCSDT) approach has been used to obtain static benchmark results and the CC3 model with an approximate treatment of triple excitations to obtain frequency-dependent results. In the case of neon the effect of excitations beyond triples has also been estimated via coupled cluster calculations including quadruple excitations (CCSDTQ), pentuple excitations (CCSDTQP), etc. up to the full configuration-interaction level. The results obtained for the anisotropy of the hypermagnetizability Deltaeta(omega), the molecular property that determines the magnetic field induced birefringence of spherically symmetric systems, are Deltaeta=2.89 a.u. for neon and Deltaeta=24.7 a.u. for argon, with a negligible effect of frequency dispersion. For neon we could estimate an absolute error on Deltaeta of 0.1 a.u. The accuracy of these results surpasses that of recently reported experimental data.

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PubMed2004

On the nature of the low-lying singlet states of 4-(Dimethyl-amino)benzonitrile.

4-(N,N-Dimethyl-amino)benzonitrile (DMABN) is a prototype molecule for dual fluorescence. The anomalous emission has been attributed to an intramolecular charge-transfer (ICT) state, and the structure of the latter is still subject to some controversy. We applied a recently developed analytical gradient code for the approximate coupled-cluster singles-and-doubles method CC2 in combination with accurate basis sets to address this problem. Fully optimized excited state structures are presented for the ICT state and the so-called locally excited state, and recent transient IR and Raman measurements on the excited states are interpreted by means of calculated harmonic frequencies. Strong evidence is found for an electronic decoupling of the phenyl and the dimethyl-amino moiety, resulting in a minimum structure for the ICT state with a twisted geometry. In contrast to previous findings, the structure of this state is, at least in the gas phase, not C(2v) symmetric but distorted towards C(s) symmetry. The distortion coordinate is a pyramidalization of the phenyl carbon atom carrying the dimethyl-amino group. The results from the CC2 model are supported by single-point calculations using more elaborate coupled-cluster models (CCSD, CCSDR(3)) and by CASSCF calculations.

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PubMed2003

Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene.

We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order Møller-Plesset perturbation theory. For the trans isomer, the planar structure with C(2h) symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet n(pi)* and (pi)(pi)* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S(1) and S(2) states agree within 0.1 eV with experimental gas-phase measurements.

Journal Article↗

PubMed2003

OPEP: a tool for the optimal partitioning of electric properties.

OPEP is a suite of FORTRAN programs targeted at the optimal partitioning of molecular electric properties. It includes an interactive module for the construction of Cartesian grids of points, on which either the molecular electrostatic potential or the induction energy is mapped. The generation of distributed multipoles and polarizabilities is achieved using either the formalism of the normal equations of the least-squares problem, which restates the fitting procedure in terms of simple matrix operations, or a statistical approach, which provides a pictorial description of the distributed models of multipoles and polarizabilities, thereby allowing the pinpointing of pathological cases. Molecular symmetry is accounted for by means of local atomic frames, which are generated in an automated fashion. A Tcl/Tk graphical user interface wraps the suite of programs, thereby making OPEP a user-friendly package for building models of distributed multipoles and polarizabilities. OPEP is an open-source suite of programs distributed free of charge under the GNU general public license (GPL) at http://www.lctn.uhp-nancy.fr/Opep.

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