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Bytautas L.,Galveston College | Scuseria G.E.,Rice University | Scuseria G.E.,King Abdulaziz University | Ruedenberg K.,Iowa State University
Journal of Chemical Physics | Year: 2015

The present study further explores the concept of the seniority number (Ω) by examining different configuration interaction (CI) truncation strategies in generating compact wave functions in a systematic way. While the role of Ω in addressing static (strong) correlation problem has been addressed in numerous previous studies, the usefulness of seniority number in describing weak (dynamic) correlation has not been investigated in a systematic way. Thus, the overall objective in the present work is to investigate the role of Ω in addressing also dynamic electron correlation in addition to the static correlation. Two systematic CI truncation strategies are compared beyond minimal basis sets and full valence active spaces. One approach is based on the seniority number (defined as the total number of singly occupied orbitals in a determinant) and another is based on an excitation-level limitation. In addition, molecular orbitals are energy-optimized using multiconfigurational-self-consistent-field procedure for all these wave functions. The test cases include the symmetric dissociation of water (6-31G), N2 (6-31G), C2 (6-31G), and Be2 (cc-pVTZ). We find that the potential energy profile for H2O dissociation can be reasonably well described using only the Ω = 0 sector of the CI wave function. For the Be2 case, we show that the full CI potential energy curve (cc-pVTZ) is almost exactly reproduced using either Ω-based (including configurations having up to Ω = 2 in the virtual-orbital-space) or excitation-based (up to single-plus-double-substitutions) selection methods, both out of a full-valence-reference function. Finally, in dissociation cases of N2 and C2, we shall also consider novel hybrid wave functions obtained by a union of a set of CI configurations representing the full valence space and a set of CI configurations where seniority-number restriction is imposed for a complete set (full-valence-space and virtual) of correlated molecular orbitals, simultaneously. We discuss the usefulness of the seniority number concept in addressing both static and dynamic electron correlation problems along dissociation paths. © 2015 AIP Publishing LLC.


Bhattacharya D.,Texas A&M University | Shil S.,North Bengal University | Misra A.,North Bengal University | Bytautas L.,Galveston College | Klein D.J.,Texas A&M University
International Journal of Quantum Chemistry | Year: 2015

The current study extends an earlier investigation (Bhattacharya, et al., Phys. Chem. Chem. Phys. 2012, 14, 6905) to further explore various photomagnetic and optical properties of bis-imino nitroxide, that is, (IN)2-based green fluorescent protein (GFP) chromophore coupled diradicals revealing new significant features. The conversion mechanisms of selected trans-isomers into their corresponding cis-conformers are discussed in detailed using a number of recently-developed density functional theory (DFT) functionals based on the Minnesota suite of DFT-models as well as using some other DFT functionals developed earlier. To provide a more in-depth analysis of variations in magnetic properties as trans-conformers (singlet ground-state) convert into their cis-analogues (triplet ground-state), the changes in exchange magnetic coupling constants J are compared with the variation of the selected aromaticity indices. The aromaticity indices include the nuclear independent chemical shift [NICS(0)] values calculated at the center of ring structures and the harmonic oscillator model of aromaticity. Furthermore, the investigation of static nonlinear optical response properties in the (IN)2-based GFP chromophore coupled diradicals reveal unusually large static first hyperpolarizabilities for these systems which is highly significant for practical applications in optics and optoelectronics. © 2015 Wiley Periodicals, Inc.


Bytautas L.,Rice University | Bytautas L.,Galveston College | Jimenez-Hoyos C.A.,Rice University | Rodriguez-Guzman R.,Rice University | Scuseria G.E.,Rice University
Molecular Physics | Year: 2014

The molybdenum dimer is an example of a transition metal system with a formal sextuple bond that constitutes a challenging case for ab initio quantum chemistry methods. In particular, the complex binding pattern in the Mo 2 molecule requires a high-quality description of non-dynamic and dynamic electron correlation in order to yield the correct shape of the potential energy curve. The present study examines the performance of a recently implemented multi-component symmetry projected Hartree-Fock (HF) approach. In this work, the spin and spatial symmetries of a trial wavefunction written in terms of non-orthogonal Slater determinants are deliberately broken and then restored in a variation-after-projection framework. The resulting symmetry-projected HF wavefunctions, which possess well-defined quantum numbers, can account for static and some dynamic correlations. A single symmetry-projected configuration in a D∞h S-UHF or a D ∞hKS-UHF framework offers a reasonable description of the potential energy curve of Mo2, though the binding energy is too small for the former. Our multi-component strategy offers a way to improve on the single configuration result in a systematic way towards the exact wavefunction: in the def2-TZVP basis set considered in this study, a 7-determinant multi-component D∞hS-UHF approach yields a bond length of 2.01 Å, in good agreement with experimental results, while the predicted binding energy is 39.2 mhartree. The results of this exploratory study suggest that a multi-component symmetry-projected HF stategy is a promising alternative in a high-accuracy description of the electronic structure of challenging systems. We also present and discuss some benchmark calculations based on the CEEIS-FCI (correlation energy extrapolation by intrinsic scaling-full configuration interaction) method for selected geometries. © 2014 Taylor & Francis.


Zhang X.,Indiana University | He N.,University of Texas Medical Branch | Gu D.,Indiana University | Wickliffe J.,Tulane University | And 3 more authors.
Journal of Genetics and Genomics | Year: 2015

Lung cancer causes more deaths than breast, colorectal and prostate cancers combined. Despite major advances in targeted therapy in a subset of lung adenocarcinomas, the overall 5-year survival rate for lung cancer worldwide has not significantly changed for the last few decades. DNA repair deficiency is known to contribute to lung cancer development. In fact, human polymorphisms in DNA repair genes such as xeroderma pigmentosum group C (XPC) are highly associated with lung cancer incidence. However, the direct genetic evidence for the role of XPC for lung cancer development is still lacking. Mutations of the Kirsten rat sarcoma viral oncogene homolog (Kras) or its downstream effector genes occur in almost all lung cancer cells, and there are a number of mouse models for lung cancer with these mutations. Using activated Kras, KrasLA1, as a driver for lung cancer development in mice, we showed for the first time that mice with KrasLA1 and Xpc knockout had worst outcomes in lung cancer development, and this phenotype was associated with accumulated DNA damage. Using cultured cells, we demonstrated that induced expression of oncogenic KRASG12V led to increased levels of reactive oxygen species (ROS) as well as DNA damage, and both can be suppressed by anti-oxidants. Our results suggest that XPC may help repair DNA damage caused by KRAS-mediated production of ROS. © 2015 Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, and Genetics Society of China.


Bytautas L.,Rice University | Bytautas L.,Galveston College
Croatica Chemica Acta | Year: 2013

Dimethylmercury (CH3-Hg-CH3) and other Hg-containing compounds can be found in atmospheric and aqueous environments. These substances are highly toxic and pose a serious environmental and health hazard. Therefore, the understanding of chemical processes that affect the stability of these substances is of great interest. The mercury-containing compounds can be detected in atmosphere, as well as soil and aqueous environments where, in addition to water molecules, numerous ionic species are abundant. In this study we explore the stability of several small, Hg-containing compounds with respect to water molecules, hydronium (H3O+) ions as well as other small molecules/ions using density functional theory and wave function quantum chemistry methods. It is found that the stability of such molecules, most notably of dimethylmercury, can be strongly affected by the presence of the hydronium H3O+ ions. Although the present theoretical study represents gas phase results, it implies that pH level of a solution should be a major factor in determining the degree of abundance for dimethylmercury in aqueous environment. In particular, it is found that CH 3-Hg-CH3 reacts readily with the H3O + ion producing CH3-Hg-OH2 + and methane indicating that low-pH levels favor the decomposition of dimethylmercury. On the other hand, our study suggests that high-pH levels in aqueous environment would favor stronglybound complexes of [CH 3-Hg-CH3•OH]- species. Overall, the theoretical evidence presented in this study offers an explanation for the available experimental data concerning the stability of dimethylmercury and other mercury-containing compounds having the general structure X-Hg-Y (X,Y = CH3 and Cl) with respect to various ligands L (L = H2O, NH3, H3O+, OH-, Cl- and NH4 +).

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