First principles electron-correlated calculations of optical absorption in magnesium clusters

In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of various isomers of magnesium clusters Mg$_{n}$ (n=2--5), corresponding to valence transitions. Geometry optimization of several low-lying isomers of each cluster was carried out using coupled-cluster singles doubles (CCSD) approach, and these geometries were subsequently employed to perform ground and excited state calculations using either the full-CI (FCI) or the multi-reference singles-doubles configuration interaction (MRSDCI) approach, within the frozen-core approximation. Our calculated photoabsorption spectrum of magnesium dimer (Mg$_{2}$) isomer is in excellent agreement with the experiments both for peak positions, and intensities. Owing to the sufficiently inclusive electron-correlation effects, these results can serve as benchmarks against which future experiments, as well as calculations performed using other theoretical approaches, can be tested.Comment: 23 pages and 21 figures of main tex

STATISTICAL MEDIA OPTIMIZATION FOR LUTEIN PRODUCTION FROM MICROALGAE Auxenochlorella protothecoides SAG 211-7A

In this study, the heterotrophic production potential of the secondary carotenoid lutein by the green microalgae Auxenochlorella protothecoides SAG 211-7a was investigated. A sequential statistical technique was applied to optimize modified bold’s basal media (MBB) to enhance the lutein production from microalgae Auxenochlorella protothecoides SAG 211-7a. Taguchi orthogonal array method was applied to select the various independent variables which affect the lutein production. It showed that sucrose, yeast extract, MgSO4.7H2O and EDTA were the significant factors affect the lutein production. Further, to increase the lutein yield and to study the interaction between these factors response surface methodology (RSM) was employed. The statistical model was validated with respect to lutein production under the conditions predicted by the model containing sucrose 14.0 g/l, yeast extract 3.0 g/l, MgSO4.7H2O 0.8 g/l and EDTA 0.76 g/l. The production of lutein obtained experimentally using the above medium was 1303 ± 25.32 μg/l, which is in correlation with the predicted value of 1337.21 g/l by the RSM regression study. Thus after sequential statistical media optimization strategy a 5-fold enhancement in lutein production was achieved

Remarkable Hydrogen Storage on Beryllium Oxide Clusters: First Principles Calculations

Since the current transportation sector is the largest consumer of oil, and subsequently responsible for major air pollutants, it is inevitable to use alternative renewable sources of energies for vehicular applications. The hydrogen energy seems to be a promising candidate. To explore the possibility of achieving a solid-state high-capacity storage of hydrogen for onboard applications, we have performed first principles density functional theoretical calculations of hydrogen storage properties of beryllium oxide clusters (BeO)$_{n}$ (n=2 -- 8). We observed that polar BeO bond is responsible for H$_{2}$ adsorption. The problem of cohesion of beryllium atoms does not arise, as they are an integral part of BeO clusters. The (BeO)$_{n}$ (n=2 -- 8) adsorbs 8--12 H$_{2}$ molecules with an adsorption energy in the desirable range of reversible hydrogen storage. The gravimetric density of H$_{2}$ adsorbed on BeO clusters meets the ultimate 7.5 wt% limit, recommended for onboard practical applications. In conclusion, beryllium oxide clusters exhibit a remarkable solid-state hydrogen storage.Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in JPCC, copyright American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see , see http://pubs.acs.org/doi/abs/10.1021/jp410994

Optical Absorption in B13_{13} Cluster: A Time-Dependent Density Functional Approach

The linear optical absorption spectra of three isomers of planar boron cluster B$_{13}$ are calculated using time-dependent spin-polarized density functional approach. The geometries of these cluster are optimized at the B3LYP/6-311+G* level of theory. Even though the isomers are almost degenerate, the calculated spectra are quite different, indicating a strong structure-property relationship. Therefore, these computed spectra can be used in the photo-absorption experiments to distinguish between different isomers of a cluster.Comment: Version2: Latex and hyperref enabled. Minor typos corrected. 4 figures, 2 pages. Accepted manuscript. To appear in AIP Conference Proceeding

Large-scale first principles configuration interaction calculations of optical absorption in boron clusters

We have performed systematic large-scale all-electron correlated calculations on boron clusters B$_{n}$(n=2--5), to study their linear optical absorption spectra. Several possible isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles doubles (CCSD) level of theory. Using the optimized ground-state geometries, excited states of different clusters were computed using the multi-reference singles-doubles configuration-interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wave functions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, eventually leading to their linear absorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion were carefully examined. The contribution of configurations to many body wavefunction of various excited states suggests that the excitations involved are collective, plasmonic type.Comment: 25 page, 13 figures (included); Invited article for a special issue titled "Theoretical Simulation and Computational Design of Nanomaterials and Biomaterials", of the journal Nano Lif

Pressure-Induced Topological Phase Transitions in CdGeSb2_2 and CdSnSb2_2

Topological quantum phase transitions (TQPTs) in a material induced by external perturbations are often characterized by band touching points in the Brillouin zone. The low-energy excitations near the degenerate band touching points host different types of fermions while preserving the topological protection of surface states. An interplay of different tunable topological phases offers an insight into the evolution of the topological character. In this paper, we study the occurrence of TQPTs as a function of hydrostatic pressure in CdGeSb$_2$ and CdSnSb$_2$ chalcopyrites, using the first-principles calculations. At ambient pressure, both materials are topological insulators having a finite band gap with inverted order of Sb-$s$ and Sb-$p_x$,$p_y$ orbitals of valence bands at the $\Gamma$ point. On the application of hydrostatic pressure the band gap reduces, and at the critical point of the phase transition, these materials turn into Dirac semimetals. On further increasing the pressure beyond the critical point, the band inversion is reverted making them trivial insulators. The pressure-induced change in band topology from non-trivial to trivial phase is also captured by L\"{u}ttinger model Hamiltonian calculations. Our model demonstrates the critical role played by a pressure-induced anisotropy in frontier bands in driving the phase transitions. These theoretical findings of peculiar coexistence of multiple topological phases in the same material provide a realistic and promising platform for the experimental realization of the TQPT.Comment: 7 pages, 7 figure