The accuracy of first-order density matrices obtained by electronic structure methods involving the valence electrons only
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Euwema, R. N.↗
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Theoretical Study of Decomposition Pathways for HArF and HKrF
To provide theoretical insights into the stability and dynamics of the new rare gas compounds HArF and HKrF, reaction paths for decomposition processes HRgF to Rg + HF and HRgF to H + Rg + F (Rg = Ar, Kr) are calculated using ab initio electronic structure methods. The bending channels, HRgF to Rg + HF, are described by single-configurational MP2 and CCSD(T) electronic structure methods, while the linear decomposition paths, HRgF to H + Rg + F, require the use of multi-configurational wave functions that include dynamic correlation and are size extensive. HArF and HKrF molecules are found to be energetically stable with respect to atomic dissociation products (H + Rg + F) and separated by substantial energy barriers from Rg + HF products, which ensure their kinetic stability. The results are compatible with experimental data on these systems.
Chaban, Galina M.↗
Research in Computational Astrobiology
We report on several projects in the field of computational astrobiology, which is devoted to advancing our understanding of the origin, evolution and distribution of life in the Universe using theoretical and computational tools. Research projects included modifying existing computer simulation codes to use efficient, multiple time step algorithms, statistical methods for analysis of astrophysical data via optimal partitioning methods, electronic structure calculations on water-nuclei acid complexes, incorporation of structural information into genomic sequence analysis methods and calculations of shock-induced formation of polycylic aromatic hydrocarbon compounds.
Chaban, Galina↗
Nanomechanics of Carbon and CxByNz Nanotubes: Via a Quantum Molecular Dynamics Method
Nanomechanics of single-wall C, BN and BC$_3$ and B doped C nanotubes under axial compression and tension are investigated through a generalized tight-binding molecular dynamics (GTBMD) and {\it ab-initio} electronic structure methods. The dynamic strength of BN, BC$_3$ and B doped C nanotubes for small axial strain are comparable to each other. The main difference is in the critical strain at which structural collapse occurs. For example, even a shallow doping with B lowers the value of critical strain for C nanotubes. The critical strain for BN nanotube is found to be more than that for the similar C nanotube. Once the structural collapse starts to occur we find that carbon nanotubes irreversibly go into plastic deformation regime via the formation of tetrahedral (four-fold coordinated) bonds at the location of sharp pinches or kinks. This finding is considerably different from the classical MD (molecular dynamics) simulation results known so far. The energetics and electronic densities of states of the collapsed structures, investigated with {\it ab-initio) methods, will also be discussed.
Srivastava, Deepak↗
Theoretical determination of chemical rate constants using novel time-dependent methods
The work completed within the grant period 10/1/91 through 12/31/93 falls primarily in the area of reaction dynamics using both quantum and classical mechanical methodologies. Essentially four projects have been completed and have been or are in preparation of being published. The majority of time was spent in the determination of reaction rate coefficients in the area of hydrocarbon fuel combustion reactions which are relevant to NASA's High Speed Research Program (HSRP). These reaction coefficients are important in the design of novel jet engines with low NOx emissions, which through a series of catalytic reactions contribute to the deterioration of the earth's ozone layer. A second area of research studied concerned the control of chemical reactivity using ultrashort (femtosecond) laser pulses. Recent advances in pulsed-laser technologies have opened up a vast new field to be investigated both experimentally and theoretically. The photodissociation of molecules adsorbed on surfaces using novel time-independent quantum mechanical methods was a third project. And finally, using state-of-the-art, high level ab initio electronic structure methods in conjunction with accurate quantum dynamical methods, the rovibrational energy levels of a triatomic molecule with two nonhydrogen atoms (HCN) were calculated to unprecedented levels of agreement between theory and experiment.
Dateo, Christopher E.↗
Computing Highly Accurate Spectroscopic Line Lists for Characterization of Planetary Atmospheres: CO2 and SO2 Line Lists Needed for Modeling Venus
Over the last decade, it has become apparent that the most effective approach for determining highly accurate rotational and rovibrational line lists for molecules of interest in planetary atmospheres and other astrophysical environments is through a combination of highresolution laboratory experiments coupled with state-of-the art ab initio quantum chemistry methods. The approach involves computing the most accurate potential energy surface (PES) possible using state-of-the art electronic structure methods, followed by computing rotational and rovibrational energy levels using an exact variational method to solve the nuclear Schrödinger equation. Then, reliable experimental data from high-resolution experiments is used to refine the ab initio PES in order to improve the accuracy of the computed energy levels and transition energies. From the refinement step, we have been able to achieve an accuracy of approximately 0.015 cm-1 for rovibrational transition energies, and even better for purely rotational transitions. This combined "experiment / theory" approach allows for determination of essentially a complete line list, with hundreds of millions of transitions, and having the transition energies and intensities be highly accurate. Our group has successfully applied this approach to determine highly accurate line lists for NH3, CO2 and isotopologues, and SO2 and isotopologues. Here I will report our latest results for CO2 and SO2 including all isotopologues. Comparisons to the available data in HITRAN2012 and other available databases will be shown, though we note that our line lists for SO2 are significantly more complete than any other databases. Since it is important to span a large temperature range in order to model the spectral signature of Venus as well as exoplanets, we will demonstrate how the spectra change on going from low temperatures (100 K) to higher temperatures (500 K to 1500 K).
potential energy surface (PES)5405 PLANETARY SCIEN↗
Gas-Phase Reaction Pathways and Rate Coefficients for the Dichlorosilane-Hydrogen and Trichlorosilane-Hydrogen Systems
As part of NASA Ames Research Center's Integrated Process Team on Device/Process Modeling and Nanotechnology our goal is to create/contribute to a gas-phase chemical database for use in modeling microelectronics devices. In particular, we use ab initio methods to determine chemical reaction pathways and to evaluate reaction rate coefficients. Our initial studies concern reactions involved in the dichlorosilane-hydrogen (SiCl2H2--H2) and trichlorosilane-hydrogen (SiCl2H-H2) systems. Reactant, saddle point (transition state), and product geometries and their vibrational harmonic frequencies are determined using the complete-active-space self-consistent-field (CASSCF) electronic structure method with the correlation consistent polarized valence double-zeta basis set (cc-pVDZ). Reaction pathways are constructed by following the imaginary frequency mode of the saddle point to both the reactant and product. Accurate energetics are determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations (CCSD(T)) extrapolated to the complete basis set limit. Using the data from the electronic structure calculations, reaction rate coefficients are obtained using conventional and variational transition state and RRKM theories.
Dateo, Christopher E.↗
Ab initio characterization of ClOOH - Implications for atmospheric chemistry
The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of ClOOH are determined using the CCSD(T) (singles and doubles coupled-cluster theory plus a perturbational estimate of the effects of connected triple excitations) electronic structure method in conjunction with a TZ2P (triple xi plus double polarization) basis set. The heat of formation of CIOOH is determined (using two different isodesmic reactions) to be +1.5 +/- 1 kcal/mol at 0 K or +0.2 +/- 1 kcal/mol at 298.15 K. Using the computed heat of formation, we examined the stability of ClOOH with respect to the ClO + OH, ClOO + H, and HOO + Cl dissociation limits. Since ClOOH is found to be quite stable, it is argued that the chemistry of ClOOH should be included in any accurate modeling of the stratosphere.
Lee, Timothy J.↗
The (FHCl)- molecular anion - Structural aspects, global surface, and vibrational eigenspectrum
State of the art ab initio electronic structure methods have been used to investigate the (FHCl)- molecular anion. It is proposed that the geometric structure and binding energies of the complex are r(e)(H-F) = 0.963 +/- 0.003 A, R(e)(H-Cl) = 1.925 +/- 0.015 A, and D0(HF + Cl(-)) = 21.8 +/- 0.4 kcal/mol. A Morokuma decomposition of the ion-molecular bonding give the following electrostatic, polarization, exchange repulsion, dispersion, and charge-transfer plus higher-order mixing components of the vibrationless complexation energy: -27.3, -5.2, +18.3, -4.5, and -5.0 kcal/mol, respectively. A couples cluster single and doubles global surface is constructed from 208 and 228 energy points for linear and bent configurations, respectively, these being fit to rms errors of only 3.9 and 9.3/cm, respectively, below 8000/cm. Converged J = 0 and J = 1 variational eigenstates of the (FHCl)- surface to near the HF + Cl(-) dissociation threshold are determined. The fundamental vibrational frequencies are found to be nu1 = 247/cm, nu2 = 876/cm, and nu3 = 2884/cm. The complete vibrational eigenspectrum is analyzed.
Klepeis, Neil E.↗
Nano-Plasticity of Single-Wall Carbon Nanotubes Under Uniaxial Compression
Nano-plasticity of thin single-wall carbon nanotubes under uniaxial compression is investigated through generalized tight-binding molecular dynamics (GTBMD) and ab-initio electronic structure methods. A novel mechanism of nano-plasticity of carbon nanotubes under uniaxial compression is observed in which bonding geometry collapses from a graphitic (sp(sup 2)) to a localized diamond like (sp(sup 3)) reconstruction. The computed critical stress (approximately equals 153 G Pa) and the shape of the resulting plastic deformation is in good agreement with recent experimental observation of collapse and fracture of compressed carbon nanotubes in polymer composites.
Srivastava, Deepak↗
Ab Initio and Improved Empirical Potentials for the Calculation of the Anharmonic Vibrational States and Intramolecular Mode Coupling of N-Methylacetamide
The second-order Moller-Plesset ab initio electronic structure method is used to compute points for the anharmonic mode-coupled potential energy surface of N-methylacetamide (NMA) in the trans(sub ct) configuration, including all degrees of freedom. The vibrational states and the spectroscopy are directly computed from this potential surface using the Correlation Corrected Vibrational Self-Consistent Field (CC-VSCF) method. The results are compared with CC-VSCF calculations using both the standard and improved empirical Amber-like force fields and available low temperature experimental matrix data. Analysis of our calculated spectroscopic results show that: (1) The excellent agreement between the ab initio CC-VSCF calculated frequencies and the experimental data suggest that the computed anharmonic potentials for N-methylacetamide are of a very high quality; (2) For most transitions, the vibrational frequencies obtained from the ab initio CC-VSCF method are superior to those obtained using the empirical CC-VSCF methods, when compared with experimental data. However, the improved empirical force field yields better agreement with the experimental frequencies as compared with a standard AMBER-type force field; (3) The empirical force field in particular overestimates anharmonic couplings for the amide-2 mode, the methyl asymmetric bending modes, the out-of-plane methyl bending modes, and the methyl distortions; (4) Disagreement between the ab initio and empirical anharmonic couplings is greater than the disagreement between the frequencies, and thus the anharmonic part of the empirical potential seems to be less accurate than the harmonic contribution;and (5) Both the empirical and ab initio CC-VSCF calculations predict a negligible anharmonic coupling between the amide-1 and other internal modes. The implication of this is that the intramolecular energy flow between the amide-1 and the other internal modes may be smaller than anticipated. These results may have important implications for the anharmonic force fields of peptides, for which N-methylacetamide is a model.
Gregurick, Susan K.↗
Computational Study of Field Initiated Surface Reactions for Synthesis of Diamond and Silicon
This project involves using quantum chemistry to simulate surface chemical reactions in the presence of an electric field for nanofabrication of diamond and silicon. A field delivered by a scanning tunneling microscope (STM) to a nanometer scale region of a surface affects chemical reaction potential energy surfaces (PES) to direct atomic scale surface modification to fabricate sub-nanometer structures. Our original hypothesis is that the applied voltage polarizes the charge distribution of the valence electrons and that these distorted molecular orbitals can be manipulated with the STM so as to change the relative stabilities of the electronic configurations over the reaction coordinates and thus the topology of the PES and reaction kinetics. Our objective is to investigate the effect of applied bias on surface reactions and the extent to which STM delivered fields can be used to direct surface chemical reactions on an atomic scale on diamond and silicon. To analyze the fundamentals of field induced chemistry and to investigate the application of this technique for the fabrication of nanostructures, we have employed methods capable of accurately describing molecular electronic structure. The methods we employ are density functional theory (DFT) quantum chemical (QC) methods. To determine the effect of applied bias on surface reactions we have calculated the QC PESs in various applied external fields for various reaction steps for depositing or etching diamond and silicon. We have chosen reactions which are thought to play a role in etching and the chemical vapor deposition growth of Si and diamond. The PESs of the elementary reaction steps involved are then calculated under the applied fields, which we vary in magnitude and configuration. We pay special attention to the change in the reaction barriers, and transition state locations, and search for low energy reaction channels which were inaccessible without the applied bias.
Musgrave, Charles Bruce↗
Comparison of Conjugate Gradient Density Matrix Search and Chebyshev Expansion Methods for Avoiding Diagonalization in Large-Scale Electronic Structure Calculations
We report a comparison of two linear-scaling methods which avoid the diagonalization bottleneck of traditional electronic structure algorithms. The Chebyshev expansion method (CEM) is implemented for carbon tight-binding calculations of large systems and its memory and timing requirements compared to those of our previously implemented conjugate gradient density matrix search (CG-DMS). Benchmark calculations are carried out on icosahedral fullerenes from C60 to C8640 and the linear scaling memory and CPU requirements of the CEM demonstrated. We show that the CPU requisites of the CEM and CG-DMS are similar for calculations with comparable accuracy.
Bates, Kevin R.↗
Scanning tunneling microscopy characterization of the geometric and electronic structure of hydrogen-terminated silicon surfaces
Scanning tunneling microscopy (STM) methods are used to characterize hydrogen-terminated Si surfaces prepared by a novel method. The surface preparation method is used to expose the Si-SiO2 interface. STM images directly reveal the topographic structure of the Si-SiO2 interface. The dependence of interface topography on oxide preparation conditions observed by STM is compared to the results of conventional surface characterization methods. Also, the electronic structure of the hydrogen-terminated surface is studied by STM spectroscopy. The near-ideal electronic structure of this surface enables direct tunnel spectroscopy measurements of Schottky barrier phenomena. In addition, this method enables probing of semiconductor subsurface properties by STM.
Kaiser, W. J.↗
Curing Composite Materials Using Lower-Energy Electron Beams
In an improved method of fabricating composite-material structures by laying up prepreg tapes (tapes of fiber reinforcement impregnated by uncured matrix materials) and then curing them, one cures the layups by use of beams of electrons having kinetic energies in the range of 200 to 300 keV. In contrast, in a prior method, one used electron beams characterized by kinetic energies up to 20 MeV. The improved method was first suggested by an Italian group in 1993, but had not been demonstrated until recently. With respect to both the prior method and the present improved method, the impetus for the use of electron- beam curing is a desire to avoid the high costs of autoclaves large enough to effect thermal curing of large composite-material structures. Unfortunately, in the prior method, the advantages of electron-beam curing are offset by the need for special walls and ceilings on curing chambers to shield personnel from x rays generated by impacts of energetic electrons. These shields must be thick [typically 2 to 3 ft (about 0.6 to 0.9 m) if made of concrete] and are therefore expensive. They also make it difficult to bring large structures into and out of the curing chambers. Currently, all major companies that fabricate composite-material spacecraft and aircraft structures form their layups by use of automated tape placement (ATP) machines. In the present improved method, an electron-beam gun is attached to an ATP head and used to irradiate the tape as it is pressed onto the workpiece. The electron kinetic energy between 200 and 300 keV is sufficient for penetration of the ply being laid plus one or two of the plies underneath it. Provided that the electron-beam gun is properly positioned, it is possible to administer the required electron dose and, at the same time, to protect personnel with less shielding than is needed in the prior method. Adequate shielding can be provided by concrete walls 6 ft (approximately equal to 1.8 m) high and 16 in. (approximately equal to 41 cm) thick, without a ceiling. The success of the present method depends on the use of a cationic epoxy as the matrix material in the prepreg tape, heating the prepreg tape to a temperature of 50 C immediately prior to layup, and exposing the workpiece to an electron-beam dose of approximately 2 Mrad. Experiments have shown that structures fabricated by the present method have the same mechanical properties as those of nominally identical structures fabricated by the prior method with electron beams of 3 to 4 MeV.
Byrne, Catherine A.↗
Studies in electron phenomena in MOS structures: The pulsed C-V method
The pulse hysteresis capacitance voltage (C-V) provides a straight forward technique for measuring the change of various charges in MOS structures and a tool for investigating the kinetics of various electron phenomena is developed and described. The method can be used for measuring the energy distribution and kinetics of surface states with the resolution of about 1/5 x 10 to the -9 power cm eV. Some transients in an MOS structure, particularly, the thermal generation of minority charge carriers via surface states and the relaxation of minority charge carriers supplied from the inversion layer outside the MOS structure are theoretically investigated. Analytical expressions which clearly present the physics of those electron phenomena are derived.
Kaplan, G.↗
Symmetry and equivalence restrictions in electronic structure calculations
A simple method for obtaining MCSCF orbitals and CI natural orbitals adapted to degenerate point groups, with full symmetry and equivalnece restrictions, is described. Among several advantages accruing from this method are the ability to perform atomic SCF calculations on states for which the SCF energy expression cannot be written in terms of Coulomb and exchange integrals over real orbitals, and the generation of symmetry-adapted atomic natural orbitals for use in a recently proposed method for basis set contraction.
Bauschlicher, Charles W., Jr.↗
Formation of Carbon Nanotube Based Gears: Quantum Chemistry and Molecular Mechanics Study of the Electrophilic Addition of o-Benzyne to Fullerenes, Graphene, and Nanotubes
Considerable progress has been made in recent years in chemical functionalization of fullerene molecules. In some cases, the predominant reaction products are different from those obtained (using the same reactants) from polycyclic aromatic hydrocarbons (PAHs). One such example is the cycloaddition of o-benzyne to C60. It is well established that benzyne adds across one of the rings in naphthalene, anthracene and other PAHs forming the [2+4] cycloaddition product (benzobicyclo[2.2.2.]-octatriene with naphthalene and triptycene with anthracene). However, Hoke et al demonstrated that the only reaction path for o-benzyne with C60 leads to the [2+2] cycloaddition product in which benzyne adds across one of the interpentagonal bonds (forming a cyclobutene ring in the process). Either reaction product results in a loss of aromaticity and distortion of the PAH or fullerene substrate, and in a loss of strain in the benzyne. It is not clear, however, why different products are preferred in these cases. In the current paper, we consider the stability of benzyne-nanotube adducts and the ability of Brenner's potential energy model to describe the structure and stability of these adducts. The Brenner potential has been widely used for describing diamondoid and graphitic carbon. Recently it has also been used for molecular mechanics and molecular dynamics simulations of fullerenes and nanotubes. However, it has not been tested for the case of functionalized fullerenes (especially with highly strained geometries). We use the Brenner potential for our companion nanogear simulations and believe that it should be calibrated to insure that those simulations are physically reasonable. In the present work, Density Functional theory (DFT) calculations are used to determine the preferred geometric structures and energetics for this calibration. The DFT method is a kind of ab initio quantum chemistry method for determining the electronic structure of molecules. For a given basis set expansion, it is comparable in accuracy to the MP2 method (better than Hartree Fock, but less accurate than more extensive electron correlation methods such as MP4 or CCSD). However, for systems with large numbers of basis functions it more efficient than any other methods that include electron correlation effects. In this presentation we show the results of DFT calculations for the reaction of benzyne with naphthalene, C60, and nanotube models. We compare energies for [2+2] and [2+4] cycloaddition products. The preferred products for the naphthalene and C60 reactions have been determined by experiment and, thus, these cases serve as a validation of our quantum chemical approach. We also compare the DFT and Brenner potential results. Finally we can predict the likelihood of reaction between benzyne and nanotubes.
Jaffe, Richard↗