Interfacing the Ab Initio Multiple Spawning Method with Electronic Structure Methods in GAMESS: Photodecay of trans-Azomethane

This work presents a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane, using the ab initio multiple spawning (AIMS) program that has been interfaced with the General Atomic and Molecular Electronic Structure System (GAMESS) quantum chemistry package for on-the-fly electronic structure evaluation. The interface strategy is discussed, and the capabilities of the combined programs are demonstrated with a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane. Energies, gradients, and nonadiabatic coupling matrix elements were obtained with the state-averaged complete active space self-consistent field method, as implemented in GAMESS. The influence of initial vibrational excitation on the outcome of the photoinduced isomerization is explored. Increased vibrational excitation in the CNNC torsional mode shortens the excited state lifetime. Depending on the degree of vibrational excitation, the excited state lifetime varies from ∼60–200 fs. These short lifetimes are in agreement with time-resolved photoionization mass spectroscopy experiments

Design and Implementation of Scientific Software Components to Enable Multiscale Modeling: The Effective Fragment Potential (QM/EFP) Method

The design and development of scientific software components to provide an interface to the effective fragment potential (EFP) methods are reported. Multiscale modeling of physical and chemical phenomena demands the merging of software packages developed by research groups in significantly different fields. Componentization offers an efficient way to realize new high performance scientific methods by combining the best models available in different software packages without a need for package readaptation after the initial componentization is complete. The EFP method is an efficient electronic structure theory based model potential that is suitable for predictive modeling of intermolecular interactions in large molecular systems, such as liquids, proteins, atmospheric aerosols, and nanoparticles, with an accuracy that is comparable to that of correlated ab initio methods. The developed components make the EFP functionality accessible for any scientific component-aware software package. The performance of the component is demonstrated on a protein interaction model, and its accuracy is compared with results obtained with coupled cluster methods

Effect of covalent links on the structure, spectra, and redox properties of myeloperoxidase - A density functional study.

The enzyme myeloperoxidase shows several unusual properties compared to other peroxidases, e.g. a red-shifted absorption spectrum and a peroxidase activity towards chloride. It has been suggested that this is caused by the unusual covalent links between the heme group and the surrounding protein, but whether it is caused by the two ester links to Glu-242 and Asp-94 or the sulfonium ion linkage to Met-243 is unclear. To investigate these suggestions, we have used density functional theory to study the structure, spectra, and reduction potential of 25 models of myeloperoxidase in the reduced (Fe(II)) and oxidized (Fe(III)) states, as well as in the compound I (formally Fe(V)O) and II (Fe(IV)O or Fe(IV)OH) states, using appropriate models of the linkages to the Asp, Glu, and Met residues (including the back-bone connection between Glu-242 and Met-243) in varying combinations. The calculated spectral shifts indicate that both the ester and sulfonium linkages play a role in the spectral shift. On the other hand, the sulfonium linkage seems to be mainly responsible for the high positive reduction potential for the both ferric/ferrous and compound I/II couples of myeloperoxidase

Structural and photoluminescence properties of excited state intramolecular proton transfer capable compounds - Potential emissive and electron transport materials

Electronic factors influencing the photoluminescence properties and rates of excited state intramolecular proton transfer (ESIPT) reaction of o-hydroxy derivatives of 2,5-diphenyl-1,3,4-oxadiazole have been studied. The potential of these molecules as emissive and electron transport materials in designing improved organic light emitting diodes (OLEDs) has been studied by analyzing possible reasons for the unusually high Stokes shifts and ESIPT reaction rates. Time-dependent density functional theory (TDDFT) methods have been used to calculate the ground and excited state properties of the phototautomers that are the ESIPT reaction products. We study the relative effect of electron-withdrawing substituents on the proton-acceptor moiety and predict that the lowest ESIPT rate (1.9 x 10(11) s(-1)) is achieved with a dimethylamino substituent and that the Stokes shifts are around 11 000 cm(-1) for all three derivatives

Role of Electronic Curve Crossing of Benzene S-1 State in the Photodissociation of Aryl Halides, Effect of Fluorination: RASSI-SO MS-CASPT2 Study

An ab initio study of the role of electronic curve crossing of benzene S-1 state in the photo dissociation dynamics of the iodobenzene and effect of fluorination is presented. Two dissociative life times observed in iodobenzene is attributed to the coupled repulsive potential energy curves of the low-lying n-sigma*, pi-sigma*, pi-pi* states. The direct channel is attributed to the alkyl like transition and the indirect channel is attributed to the mixing of the alkyl like transitions with the low lying benzene pi-pi* transitions. Fluorination of iodobenzene results in a substantial increase in the direct channel product. To analyze the possible role of electronic curve crossing of these transitions, potential energy curves of low-lying n-sigma*, pi-sigma*, pi-pi* states were studied including spin-orbit and relativistic effects using the Restricted Active Space state interaction multistate complete active space perturbation theory (RASSI-MS-CASPT2) method. Crossing behavior of spin-free and spin-orbit potential energy curves was analyzed for the role of the benzene S-1 state. Our results indicate the curve crossing region to be around 2.00-2.35 angstrom for both C6H5I and C6F5I. Analysis of effect of fluorination on the energies of states corresponding to benzene pi-pi* and n-sigma* transitions suggests an increase in the energy of benzene pi-pi* states and a decrease in the energy of the states corresponding to n-sigma* transitions. Increased spin-orbit gap, increased separation of the benzene S-1(pi-pi*) state and n-sigma* states in the region of curve crossing, lesser mixing of the pi-pi* and n-sigma* states, an order of magnitude decrease in the transition strength to the benzene singlet transition all contributed to the observed Substantial increase in the quantum yield of the direct channel product on fluorination of aryl halides. (c) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 1962-1974, 200

Spectral and density functional studies on the absorbance and fluorescence spectra of 2-R-5-phenyl-1,3,4-oxadiazoles and their conjugate acids

Absorption and fluorescence spectra of 2-R-5-phenyl-1,3,4-oxadiazoles (R = H, methyl, tert-butyl, trifluoromethyl, amino, phenyl, benzyl) were studied experimentally and by time dependent density functional theory (TDDFT) methods. In acidic media a substantial red shift is observed due to the presence of conjugate acid forms. Two conjugate acid forms (3H and 4H) are possible for unsymmetrically substituted oxadiazoles. Relative basicities of the two basic centers of oxadiazole ring at the S-0 and S-1 geometries were calculated using the local density descriptors approach. Substituent effects were studied by analyzing the electron density distribution in the ground and excited states. Analyzing the absorption spectra and local descriptors results, we predict 4H forms to be the dominant acid forms. Calculated emission peaks of 4H forms agree well with experimental observations. An abnormal red shift in the case of the 3H forms is attributed to the increased stabilization of the N-H bond in the 3H forms compared to the 4H forms

Implementation of the Bin Hierarchy Method for Restoring a Smooth Function from a Sampled Histogram

We present BHM, a tool for restoring a smooth function from a sampled histogram using the bin hierarchy method. The theoretical background of the method is presented in [1]. The code automatically generates a smooth polynomial spline with the minimal acceptable number of knots from the input data. It works universally for any sufficiently regular shaped distribution and any level of data quality, requiring almost no external parameter specification. It is particularly useful for large-scale numerical data analysis. This paper explains the details of the implementation and the use of the program