652 research outputs found
Algorithms versus architectures for computational chemistry
The algorithms employed are computationally intensive and, as a result, increased performance (both algorithmic and architectural) is required to improve accuracy and to treat larger molecular systems. Several benchmark quantum chemistry codes are examined on a variety of architectures. While these codes are only a small portion of a typical quantum chemistry library, they illustrate many of the computationally intensive kernels and data manipulation requirements of some applications. Furthermore, understanding the performance of the existing algorithm on present and proposed supercomputers serves as a guide for future programs and algorithm development. The algorithms investigated are: (1) a sparse symmetric matrix vector product; (2) a four index integral transformation; and (3) the calculation of diatomic two electron Slater integrals. The vectorization strategies are examined for these algorithms for both the Cyber 205 and Cray XMP. In addition, multiprocessor implementations of the algorithms are looked at on the Cray XMP and on the MIT static data flow machine proposed by DENNIS
Benchmark full configuration-interaction calculations on H2O, F- and F
Full configuration-interaction calculations are reported, and compared to other methods, for H2O at its equilibrium geometry and at two geometries with the H-O bonds stretched. Since the percentage of the self-consistent field (SCF) reference in the full configuration-interaction (FCI) wave function decreases greatly with the bond elongation, the accuracy of techniques based on a single reference do not compare well with the FCI results. However, the results from a complete active space SCF/multireference configuration-interaction (CASSCF/MRCI) treatment are in good agreement with the FCI. Correlation effects in F compared to Ne are far more similar than for F- compared to Ne, despite F- and Ne being isoelectronic. Since the importance of higher than double excitations is more important for F- than F, a very high percentage of the correlation must be obtained to accurately compute the electron affinity. In a CASSCF/MRCI treatment the higher than quadruple excitations contribute 0.02 eV to the electron affinity (EA), even for modest basis sets
Spectroscopic Constants, Abundances, and Opacities of the TiH Molecule
Using previous measurements and quantum chemical calculations to derive the
molecular properties of the TiH molecule, we obtain new values for its
ro-vibrational constants, thermochemical data, spectral line lists, line
strengths, and absorption opacities. Furthermore, we calculate the abundance of
TiH in M and L dwarf atmospheres and conclude that it is much higher than
previously thought. We find that the TiH/TiO ratio increases strongly with
decreasing metallicity, and at high temperatures can exceed unity. We suggest
that, particularly for subdwarf L and M dwarfs, spectral features of TiH near
0.52 \mic, 0.94 \mic, and in the band may be more easily measureable
than heretofore thought. The recent possible identification in the L subdwarf
2MASS J0532 of the 0.94 \mic feature of TiH is in keeping with this
expectation. We speculate that looking for TiH in other dwarfs and subdwarfs
will shed light on the distinctive titanium chemistry of the atmospheres of
substellar-mass objects and the dimmest stars.Comment: 37 pages, including 4 figures and 13 tables, accepted to the
Astrophysical Journa
Theoretical dissociation energies for ionic molecules
Ab initio calculations at the self-consistent-field and singles plus doubles configuration-interaction level are used to determine accurate spectroscopic parameters for most of the alkali and alkaline-earth fluorides, chlorides, oxides, sulfides, hydroxides, and isocyanides. Numerical Hartree-Fock (NHF) calculations are performed on selected systems to ensure that the extended Slater basis sets employed for the diatomic systems are near the Hartree-Fock limit. Extended Gaussian basis sets of at least triple-zeta plus double polarization equality are employed for the triatomic system. With this model, correlation effects are relatively small, but invariably increase the theoretical dissociation energies. The importance of correlating the electrons on both the anion and the metal is discussed. The theoretical dissociation energies are critically compared with the literature to rule out disparate experimental values. Theoretical (sup 2)Pi - (sup 2)Sigma (sup +) energy separations are presented for the alkali oxides and sulfides
Line Intensities and Molecular Opacities of the FeH Transition
We calculate new line lists and opacities for the
transition of FeH. The 0-0 band of this transition is responsible for the
Wing-Ford band seen in M-type stars, sunspots and brown dwarfs. The new
Einstein A values for each line are based on a high level ab initio calculation
of the electronic transition dipole moment. The necessary rotational line
strength factors (H\"onl-London factors) are derived for both the Hund's case
(a) and (b) coupling limits. A new set of spectroscopic constants were derived
from the existing FeH term values for v=0, 1 and 2 levels of the and
states. Using these constants extrapolated term values were generated for v=3
and 4 and for values up to 50.5. The line lists (including Einstein A
values) for the 25 vibrational bands with v4 were generated using a
merged list of experimental and extrapolated term values. The FeH line lists
were use to compute the molecular opacities for a range of temperatures and
pressures encountered in L and M dwarf atmospheres. Good agreement was found
between the computed and observed spectral energy distribution of the L5 dwarf
2MASS-1507.Comment: 52 pages, 3 figures, many tables, accepted for publication in the
Astrophysical Journal Supplement
The infrared spectra of very large, compact, highly symmetric, polycyclic aromatic hydrocarbons (PAHs)
The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are
determined computationally using Density Functional Theory. Trends in the band
positions and intensities as a function of PAH size, charge and geometry are
discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions
hold as with small PAHs.
This does not hold for the other features. The larger PAH cations and anions
produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron
becomes prominent and shifts slightly to the red. In addition, the average
anion peak falls slightly to the red of the average cation peak. The similarity
in behavior of the 7.8 and 8.6 micron bands with the astronomical observations
suggests that they arise from large, cationic and anionic PAHs, with the
specific peak position and profile reflecting the PAH cation to anion
concentration ratio and relative intensities of PAH size. Hence, the broad
astronomical 7.7 micron band is produced by a mixture of small and large PAH
cations and anions, with small and large PAHs contributing more to the 7.6 and
7.8 micron component respectively.
For the CH out-of-plane vibrations, the duo hydrogens couple with the solo
vibrations and produce bands that fall at wavelengths slightly different than
their counterparts in smaller PAHs. As a consequence, previously deduced PAH
structures are altered in favor of more compact and symmetric forms. In
addition, the overlap between the duo and trio bands may reproduce the
blue-shaded 12.8 micron profile.Comment: ApJ, 36 pages, 9 fig
On the electron affinity of the oxygen atom
The electron affinity (EA) of oxygen is computed to be 1.287 eV, using 2p electron full configuration-interaction (CI) wave functions expanded in a 6s5p3d2f Slater-type orbital basis. The best complete active space self-consistent field - multireference CI (CASSCF-MRCI) result including only 2p correlation is 1.263 eV. However, inclusion of 2s intrashell and 2s2p intershell correlation increases the computed EA to 1.290 at the CASSCF-MRCI level. At the full CI basis set limit, the 2s contribution to the electron affinity is estimated to be as large as 0.1 eV. This study clearly establishes the synergistic effect between the higher excitations and basis set completeness on the electron affinity when the 2s electrons are correlated
Benchmark full configuration-interaction calculations on HF and NH2
Full configuration-interaction (FCI) calculations are performed at selected geometries for the 1-sigma(+) state of HF and the 2-B(1) and 2-A(1) states of NH2 using both DZ and DZP gaussian basis sets. Higher excitations become more important when the bonds are stretched and the self-consistent field (SCF) reference becomes a poorer zeroth-order description of the wave function. The complete active space SCF - multireference configuration-interaction (CASSCF-MRCI) procedure gives excellent agreement with the FCI potentials, especially when corrected with a multi-reference analog of the Davidson correction
The 5.25 & 5.7 m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features
Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7
m (1905 and 1754 cm) that hitherto have been little studied,
but contain information about the astronomical PAH population that complements
that of the major emission bands. Here we report a study involving both
laboratory and theoretical analysis of the fundamentals of PAH spectroscopy
that produce features in this region and use these to analyze the astronomical
spectra. The ISO SWS spectra of fifteen objects showing these PAH features were
considered for this study, of which four have sufficient S/N between 5 and 6
m to allow for an in-depth analysis. All four astronomical spectra show
similar peak positions and profiles. The 5.25 m feature is peaked and
asymmetric, while the 5.7 m feature is broader and flatter. Detailed
analysis of the laboratory spectra and quantum chemical calculations show that
the astronomical 5.25 and 5.7 m bands are a blend of combination,
difference and overtone bands primarily involving CH stretching and CH in-plane
and CH out-of-plane bending fundamental vibrations. The experimental and
computational spectra show that, of all the hydrogen adjacency classes possible
on PAHs, solo and duo hydrogens consistently produce prominent bands at the
observed positions whereas quartet hydrogens do not. In all, this a study
supports the picture that astronomical PAHs are large with compact, regular
structures. From the coupling with primarily strong CH out-of-plane bending
modes one might surmise that the 5.25 and 5.7 m bands track the neutral
PAH population. However, theory suggests the role of charge in these
astronomical bands might also be important.Comment: Accepted ApJ, 40 pages in pre-print, 14 figures, two onlin
Transition metal oxides using quantum Monte Carlo
The transition metal-oxygen bond appears prominently throughout chemistry and
solid-state physics. Many materials, from biomolecules to ferroelectrics to the
components of supernova remnants contain this bond in some form. Many of these
materials' properties strongly depend on fine details of the TM-O bond and
intricate correlation effects, which make accurate calculations of their
properties very challenging. We present quantum Monte Carlo, an explicitly
correlated class of methods, to improve the accuracy of electronic structure
calculations over more traditional methods like density functional theory. We
find that unlike s-p type bonding, the amount of hybridization of the d-p bond
in TM-O materials is strongly dependant on electronic correlation.Comment: 20 pages, 4 figures, to appear as a topical review in J. Physics:
Condensed Matte
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