Toward an Improved Ground State Potential Energy Surface of Ozone

A systematic study of the ozone potential energy surface was performed by means of high level ab initio techniques. The methods include icMR-CISD and icMR-AQCC with all electrons correlated using a full valence CAS reference space and basis sets up to sextuple-ζ quality along with extrapolation to the complete basis set limit. We computed a dense 3D grid as well as 1D cuts along stretching and bending coordinates around the open (C(2v)) equilibrium structure as well as along the minimum energy path to dissociation including the transition state and the van der Waals minimum region. The detailed analysis of our results confirms earlier calculations by the Schinke group and assures that these are not biased by deficiencies of the basis set, lack of relativistic corrections, or core correlation effects. Finally, we discuss possible sources of error that may explain the remaining discrepancies compared to experimental findings

Columbus - a program system for advanced multireference theory calculations

The COLUMBUS Program System allows high-level quantum chemical calculations based on the multiconfiguration self-consistent field, multireference configuration interaction with singles and doubles, and the multireference averaged quadratic coupled cluster methods. The latter method includes size-consistency corrections at the multireference level. Nonrelativistic (NR) and spin-orbit calculations are available within multireference configuration interaction (MRCI). A prominent feature of COLUMBUS is the availability of analytic energy gradients and nonadiabatic coupling vectors for NR MRCI. This feature allows efficient optimization of stationary points and surface crossings (minima on the crossing seam). Typical applications are systematic surveys of energy surfaces in ground and excited states including bond breaking. Wave functions of practically any sophistication can be constructed limited primarily by the size of the CI expansion rather than by its complexity. A massively parallel CI step allows state-of-the art calculations with up to several billion configurations. Electrostatic embedding of point charges into the molecular Hamiltonian gives access to quantum mechanical/molecular mechanics calculations for all wave functions available in COLUMBUS. The analytic gradient modules allow on-the-fly nonadiabatic photodynamical simulations of interesting chemical and biological problems. Thus, COLUMBUS provides a wide range of highly sophisticated tools with which a large variety of interesting quantum chemical problems can be studied. (C) 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 191-199 DOI: 10.1002/wcms.2

The Accuracy of Molecular Bond Lengths Computed by Multireference Electronic Structure Methods

We compare experimental R-e values with computed R-e values for 20 molecules using three multireference electronic structure methods, MCSCF, MR-SDCI, and MR-AQCC. Three correlation-consistent orbital basis sets are used, along with complete basis set extrapolations, for all of the molecules. These data complement those computed previously with single-reference methods. Several trends are observed. The SCF R-e values tend to be shorter than the experimental values, and the MCSCF values tend to be longer than the experimental values. We attribute these trends to the ionic contamination of the SCF wave function and to the corresponding systematic distortion of the potential energy curve. For the individual bonds, the MR-SDCI R-e values tend to be shorter than the MR-AQCC values, which in turn tend to be shorter than the MCSCF values. Compared to the previous single-reference results, the MCSCF values are roughly comparable to the MP4 and CCSD methods, which are more accurate than might be expected due to the fact that these MCSCF wave functions include no extra-valence electron correlation effects. This suggests that static valence correlation effects, such as near-degeneracies and the ability to dissociate correctly to neutral fragments, play an important role in determining the shape of the potential energy surface, even near equilibrium structures. The MR-SDCI and MR-AQCC methods predict R-e values with an accuracy comparable to, or better than, the best single-reference methods (MP4, CCSD, and CCSD(T)), despite the fact that triple and higher excitations into the extra-valence orbital space are included in the single-reference methods but are absent in the multireference wave functions. The computed R-e values using the multireference methods tend to be smooth and monotonic with basis set improvement. The molecular structures are optimized using analytic energy gradients, and the timings for these calculations show the practical advantage of using variational wave functions for which the Hellmann-Feynman theorem can be exploited. (c) 2008 Elsevier B.V. All rights reserved

The problem of interoperability: a common data format for quantum chemistry codes

An XML based format is presented for representing quantities such as molecular geometries, basis sets, and similar information requiring a small amount of data. Preliminary applications using also a HDF5 based format for handling large data are also presented

Transferência de massa e secagem em leitos vibrofluidizados: uma revisão Mass transfer and drying in vibro-fluidized beds: a review

O desenvolvimento de tecnologias de processamento e equipamentos requer a utilização de novos métodos e melhor qualidade do produto processado. Dentro do processo de secagem contínua, a utilização de equipamentos que promovem incremento nos coeficientes de transferência, é a de maior interesse. O uso de energia vibracional tem sido recomendado para materiais dispersos. Assim, uma revisão da literatura sobre a transferência de massa e a secagem em leitos vibrofluidizados foi realizada, envolvendo resultados experimentais e modelagem matemática.<br>Development of processing technology and equipments requires new methods and better quality of the processed product. In the continuous drying process, utilization of equipments that promotes an increment in the transfer coefficients becomes of the major interest. The use of vibrational energy has been recommended to the dispersed materials. Such method is based on the use of vibrational energy applied to disperse media. Thus, a literature review on the mass transfer and drying in vibro-fluidized beds was carried out, showing experimental results and mathematical modeling