LiH Systems Research Articles

Modified explicitly correlated Fock‐space coupled‐cluster method for modeling confined molecular systems

AbstractA model for calculation of the ground and excited state energies, single and double electron affinities and ionization potentials of a many‐electron system confined in a cavity with a finite boundary potential is presented. Additional integrals of explicitly correlated Fock‐space coupled‐cluster method are calculated numerically with the use of conventional Gaussian basis sets on the same type of fine grids as those used for one‐electron integrals, which represents a rational and efficient tool for modeling confined systems. The method is verified by an example of H, He, and LiH systems with spherical potential and applied to describing a representative set of diverse spin and orbital states of molecule and its ions. As the size of the system and so the total number of the basis functions are increased, the accuracy of predictions also increases. In the case of , the internuclear distance is shown to be shortened with the increase in the potential wall height, while the energy differences between , , and states change only slightly. Critical cavity radii are determined, which correspond to the spontaneous ionization of and the loss of an excess electron by .

Geo-informatics Techniques for Assessing Physiological Status and Productivity of RRIC 121 Genotype of Hevea brasiliensis (Rubber) under Different Harvesting Systems

Abstract Rubber (Hevea brasiliensis) one of the main plantation crops in Sri Lanka is the only plant species cultivated commercially for natural rubber harvesting. Novel systems for harvesting have been introduced but spatial distribution of photosynthetic potential determining key factor for sustainable cultivation has not been properly explored. Use of such techniques such as Satellite Remote Sensing (RS) and Geographic Information System (GIS) to analyse spatial and biological factors related to the productivity of rubber plantation with different harvesting systems is the main objective of the present study. Quikebird high resolution satellite images were used for RS analysis. Chlorophyll content of rubber leaves was measured using a SPAD-502 chlorophyll meter. Chlorophyll content and satellite images were analysed using GIS and spatial statistical methods to determine the variation in different harvesting systems. Yield data were collected from the study site and yield parameters were correlated with chlorophyll content and Normalized Difference Vegetation Index (NDVI) values. Results revealed all systems exhibited promising yield performance without significant deviation but slightly higher yield per hectare per year (YPH) and dry rubber content of latex (DRC) were recorded in quarter spiral based once in three days (S/3 d4) and weekly (S/2 d1 2d7) harvesting systems. Chlorophyll content and rubber yield showed direct correlation in all systems. NDVI vs chlorophyll showed positive correlation r2=0.65 and spatial distribution of chlorophyll and NDVI values demonstrated sound physiological status of plants across the plantation with different harvesting systems. Cost effective LIH systems showed better production trend demonstrating relatively higher yield while reducing tapping cost and labour. Satellite based remote sensing technique is an easy and efficient tool to estimate productivity of rubber plantation over a large area. Keywords: Chlorophyll, NDVI, harvesting, Remote Sensing, Rubber.

Ternary Amides Containing Transition Metals for Hydrogen Storage: A Case Study with Alkali Metal Amidozincates.

The alkali metal amidozincates Li4 [Zn(NH2)4](NH2)2 and K2[Zn(NH2)4] were, to the best of our knowledge, studied for the first time as hydrogen storage media. Compared with the LiNH2-2 LiH system, both Li4 [Zn(NH2)4](NH2)2-12 LiH and K2[Zn(NH2)4]-8 LiH systems showed improved rehydrogenation performance, especially K2[Zn(NH2)4]-8 LiH, which can be fully hydrogenated within 30 s at approximately 230 °C. The absorption properties are stable upon cycling. This work shows that ternary amides containing transition metals have great potential as hydrogen storage materials.

Excitation energies from extended random phase approximation employed with approximate one- and two-electron reduced density matrices

Starting from Rowe's equation of motion we derive extended random phase approximation (ERPA) equations for excitation energies. The ERPA matrix elements are expressed in terms of the correlated ground state one- and two-electron reduced density matrices, 1- and 2-RDM, respectively. Three ways of obtaining approximate 2-RDM are considered: linearization of the ERPA equations, obtaining 2-RDM from density matrix functionals, and employing 2-RDM corresponding to an antisymmetrized product of strongly orthogonal geminals (APSG) ansatz. Applying the ERPA equations with the exact 2-RDM to a hydrogen molecule reveals that the resulting (1)Σ(g)(+) excitation energies are not exact. A correction to the ERPA excitation operator involving some double excitations is proposed leading to the ERPA2 approach, which employs the APSG one- and two-electron reduced density matrices. For two-electron systems ERPA2 satisfies a consistency condition and yields exact singlet excitations. It is shown that 2-RDM corresponding to the APSG theory employed in the ERPA2 equations yields excellent singlet excitation energies for Be and LiH systems, and for the N(2) molecule the quality of the potential energy curves is at the coupled cluster singles and doubles level. ERPA2 nearly satisfies the consistency condition for small molecules that partially explains its good performance.

Hydrogen sorption in MgH2-based composites: The role of Ni and LiBH4 additives

Abstract In the present work we investigate the hydrogen sorption properties of composites in the MgH2–Ni, MgH2–Ni–LiH and MgH2–Ni–LiBH4 systems and analyze why Ni addition improve hydrogen sorption rates while LiBH4 enhance the hydrogen storage capacity. Although all composites with Ni addition showed significantly improved hydrogen storage kinetics compared with the pure MgH2, the fastest hydrogen sorption kinetics is obtained for Ni-doped MgH2. The formation of Mg2Ni/Mg2NiH4 in Ni-doped MgH2 composite and its microstructure allows to uptake 5.0 wt% of hydrogen in 25 s and to release it in 8 min at 275 °C. In the MgH2–Ni–LiBH4 composite, decomposition of LiBH4 occurs during the first dehydriding leading to the formation of diborane, which has a Ni catalyst poison effect via the formation of a passivating boron layer. A combination of FTIR, XRD and volumetric measurements demonstrate that the formation of MgNi3B2 in the MgH2–Ni–LiBH4 composite happens in the subsequent hydriding cycle from the reaction between Mg2Ni/Mg2NiH4 and B. Activation energy analysis demonstrates that the presence of Ni particles has a catalytic effect in MgH2–Ni and MgH2–Ni–LiH systems, but it is practically nullified by the addition of LiBH4. The beneficial role of LiBH4 on the hydrogen storage capacity of the MgH2–Ni–LiBH4 composite is discussed.

Thermodynamic Modeling of the Li-H and Ca-H Systems

The phase diagram and thermodynamic properties of the Li-H and Ca-H systems in the literature are critically reviewed. The Gibbs energy functions of individual phases in these two systems are modeled by the CALPHAD approach. The associate solution model and substitutional model are employed to represent the thermodynamic properties of the liquid phase in the Li-H and Ca-H systems, respectively. The available experimental data are well reproduced by the present modeling. With the obtained Gibbs energy functions, the phase relationships in the Li-H and Ca-H systems at high pressures are also predicted.

Atoms in molecules: beyond Born–Oppenheimer paradigm

This contribution presents the first atoms in molecules study that goes beyond the Born–Oppenheimer paradigm employing the newly developed two-component quantum theory of atoms in molecules (TC-QTAIM). The LiH, LiD, and LiT systems containing quantum instead of clamped hydrogen nuclei are used as typical examples. The computational analysis that is done on non-adiabatic wavefunctions derived from the fully variational multicomponent molecular orbital approach (FV-MC-MO) results in hydrogen atomic basins without any clamped nucleus. The topological analysis of the Γ-field, the field that replaces the usual one-electron density used in the orthodox topological analysis, reveals delicate differences among the considered systems. The calculation of basin properties also demonstrates that the TC-QTAIM differentiates among atomic basins containing isotopes. Since the nuclear dynamics is contained intrinsically in non-adiabatic wavefunctions, the nuclear contribution to both topological analysis and basin properties naturally emerges from the TC-QTAIM analysis resolving the long-standing obstacle of consistent incorporation of nuclear dynamics within the context of the orthodox QTAIM. Also, a similar analysis is done on non-adiabatic wavefunctions describing excited instead of ground nuclear vibrations of the considered systems demonstrating the fact that TC-QTAIM is capable of being employed for both ground and excited nuclear vibrational states.

Decomposition mechanism of magnesium amide Mg(NH2)2

Electronic structure and the total energy of the Mg(NH2)2 were calculated using first principle theory. The bonding characteristics and decomposition mechanism of the Mg(NH2)2 were clarified based on the electronic structure and the total energies. The bonding interactions of the Mg atoms with the two [NH2] ligands are slightly different, while it shows a significant difference in the bonding interactions between the N and the H atoms within the [NH2] ligands. The weakest bond is the N2–H2 bond in the [NH2](2) ligand. A decomposition mechanism of the Mg(NH2)2 was proposed based on the bonding characteristics. The decomposition of the Mg(NH2)2 is performed by two steps. First H+ cations decompose from the [NH2] ligands due to their weaker bonds with the matrix, and then [NH2]− anions decompose. The H+ cations and [NH2]− anions therefore react each other to generate NH3. For the Mg(NH2)2 + LiH systems, it is most likely that the Mg(NH2) decomposes to MgNH, H+cation, and [NH2]− anion first, and then the released H+ cation and [NH2]− anion either react each other to form NH3 and then reacts with LiH, or directly react with Li+ cation and H− anion if LiH is decomposed. Both of the reactions generate the LiNH2 and the H2. And the LiNH2 further mixes with MgNH to form the LiMgN2H3. The is the first step of a multi-step dehydrogenation process of the Mg(NH2)2–LiH system [Isobe S, Ichikawa T, Leng H, Fujii H, Kojima Y. Hydrogen desorption processes in Li–Mg–N–H systems. J Phys Chem Solids 2008;69:22234.].

Projector Monte Carlo method based on configuration state functions. Test applications to the H 4 system and dissociation of LiH

A projector Monte Carlo method based on configuration state functions (PMC-CSF) is developed, which does not require the nodes of trial wave functions. The accuracy of the PMC-CSF method is systematically improved by increasing the number of walkers. As demonstrated by test applications to the H 4 and LiH systems, full-CI energies are obtainable as a limit for a given basis set. In addition, the PMC-CSF method is suitable for parallel calculations.

Dehydriding and rehydriding properties of Mg(NH 2) 2–LiH systems

The dehydriding and rehydriding properties of the mixtures of 3Mg(NH 2) 2 + nLiH ( n = 6, 8, and 12) were investigated by pressure–composition ( p– c) isotherm and X-ray diffraction (XRD) measurements in order to clarify the effects of the LiH ratio n on the properties. The amounts of the hydrogen desorbed from the mixtures with n = 6, 8, and 12 were 5.4, 5.1, and 4.5 mass%, respectively; this indicates that the amounts on a unit mass basis decrease with increasing n. However, the molar ratios of the desorbed hydrogen to the mixtures estimated from the amounts were almost equal, and also the features of the p– c isotherms were similar to each other. Moreover, the Li 2Mg(NH) 2 and LiH phases were observed in XRD profiles of all the mixtures after p– c isotherm measurements. These results suggest that the dehydriding reaction of the mixtures of 3Mg(NH 2) 2 + nLiH ( n = 6, 8, and 12) under hydrogen pressure is not dominantly affected by the value of n. On the other hand, the amounts of the ammonia desorbed from the mixtures detected by mass spectroscopy decreased with increasing n.

One‐electron properties using a CI method based on multiple Hartree–Fock solutions

AbstractA multi‐reference configuration interaction (CI) method based on multiple Hartree–Fock (HF) solutions is applied to study the permanent dipole moment of the LiH, BH, FH, and H2O systems with the minimal atomic basis. The LiH was also calculated with the double‐zeta atomic basis. This method uses several HF solutions as references to expand the state functions. For each one of the systems studied, several HF solutions were obtained in the appropriate point and spin symmetry classes, i.e., 1Σ+ and 1A1. With each of these solutions is generated a distinct basis of the same full CI space. The set of these bases is a system of generators of the full CI space and the multi‐reference HF (MRHF) bases are built mixing configuration state functions (CSFs) originated from distinct HF solutions. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Calculation of size-intensive transition moments from the coupled cluster singles and doubles linear response function

Coupled cluster singles and doubles linear response (CCLR) calculations have been carried out for excitation energies and dipole transition strengths for the lowest excitations in LiH, CH+, and C4 and the results compared with the results from a CI-like approach to equation of motion coupled cluster (EOMCC). The transition strengths are similar in the two approaches for single molecule calculations on small systems. However, the CCLR approach gives size-intensive dipole transition strengths, while the EOMCC formalism does not. Thus, EOMCC calculations can give unphysically dipole transition strengths, e.g., in EOMCC calculations on a sequence of noninteracting LiH systems we obtained a negative dipole strength for the lowest totally symmetric dipole allowed transition for 19 or more noninteracting LiH systems. The CCLR approach is shown to be a very attractive ‘‘black box’’ approach for the calculation of transition moments.

Calculation of frequency-dependent polarizabilities using coupled-cluster response theory

Coupled-cluster singles and doubles linear response (CCLR) calculations have been presented for frequency-dependent dipole polarizabilities and the results compared with the results from a CI-like approach to the equation of motion (EOMCC). The frequency-dependent polarizabilities are similar in the two approaches for single molecule calculations on small systems. However, the CCLR approach gives size-extensive polarizabilities, whereas the EOMCC approach does not. EOMCC calculations can therefore give unphysical polarizabilities, e.g. EOMCC calculations on a sequence of non-interacting LiH systems gave a negative polarizability for 20 or more non-interacting LiH systems. The CCLR approach is shown to be an attractive “black box” approach for the calculation of accurate frequency-dependent polarizabilities.

Performance of a gauge-invariant method on calculated dynamic polarizabilities.

In the calculation of the dynamic polarizability including electronic correlation and ensuring gauge invariance, contributions of spectroscopic states and quasispectral series are analyzed. An improvement of our original electric-field variant ket approach is proposed and illustrated by application to H and LiH systems. © 1992 The American Physical Society.

Perturbation theory of intermolecular interactions: What is the problem, are there solutions?

We review the nature of the problem in the framework of Rayleigh–Schrödinger perturbation theory (the polarization approximation) considering explicitly two examples: the interaction of two hydrogen atoms and the interaction of Li with H. We show, in agreement with the work of Claverie and of Morgan and Simon, that the LiH problem is dramatically different from the H2 problem. In particular, the physical states of LiH are higher in energy than an infinite number of discrete, unphysical states and they are buried in a continuum of unbound, unphysical states, which starts well below the lowest physical state. Claverie has shown that the perturbation expansion, under these circumstances, is likely to converge to an unphysical state of lower energy than the physical ground state, if it converges at all. We review, also, the application of two classes of exchange perturbation theory to LiH and larger systems. We show that the spectra of three Eisenschitz–London (EL) class, exchange perturbation theories have no continuum of unphysical states overlaying the physical states and no discrete, unphysical states below the lowest physical state. In contrast, the spectra of two Hirschfelder–Silbey class theories differ hardly at all from that found with the polarization approximation. Not one of the EL class of perturbation theories, however, eliminates all of the discrete unphysical states. The best one establishes a one-to-one correspondence between the lowest energy states of the unperturbed and perturbed Hamiltonians, and a one-to-two correspondence for the higher states. We suggest that the EL class perturbation theories would be good starting points for the development of more effective perturbation theories for intermolecular interactions.

Molecular Charge Distributions and Chemical Bonding. V. Molecular Excitation, Ionization, and Electron Attachment

The detailed changes in the electronic charge distributions upon electronic excitation, ionization, and electron attachment are studied for selected diatomic molecules. The processes considered involve the following systems and states: O2(X 3Σg−, a 1Δg, b 1Σg+), O2+(X 2Πg), O2−(2Πg); BeH and MgH (X 2Σ+, A 2Πr); OH and SH (X 2Πi, A 2Σ+), OH+ and SH+ (X 3Σ−), and OH− and SH− (X 1Σ+). It is shown that metastable excitation in the oxygen molecule produces only a very small change in the electronic charge distribution of the system. This suggests an important role for the spin density in determining the differential behavior of these systems. The A 2Πr ← X 2Σ+ and A 2Σ+ ← X 2Πi excitations for the (BeH; MgH) and (OH; SH) pairs are examined and compared to the “active” electron approximation. The changes in binding of the system associated with these various excitations, ionization, or electron attachments is examined in terms of the partial forces exerted by the various molecular orbitals. In particular, the sharp conversion to a pronounced “ionic” molecule is evident in the BeH and MgH excitations. The A 2Πr states of BeH and MgH are as ionic as the LiH and NaH systems in their ground states. It is demonstrated that the binding ability or properties of a given orbital shows a simple graduated trend in a sequence (AB+; AB; AB−) which is not strongly dependent on the particular state symmetry of the system. The binding role assigned to a particular molecular orbital in a molecule is shown to depend on the particular state and charge situation. The implications of these results towards providing a role for the excited states and molecular ions in the theory of chemical bonding are briefly discussed.

Applications of Diatomics-in-Molecules Theory. IV. Dimers of LiH

Ellison's semiempirical diatomics-in-molecules method, applied to the study of dimeric LiH systems, yields predictions of two conformations stable with respect to isolated diatomic systems. A metastable D2h diamond structure (comparable to alkali-metal halide dimers studied by Berkowitz) with a Li–H separation of 3.36 Bohr units lies 13.8 kcal/mole lower in energy than the system Li2 + H2 and 28.4 kcal/mole lower than 2 LiH. A stable C2υ structure (analogous to previously predicted disphenoid Li4 molecules) with perpendicular diatomics Li2 and H2 separated by 2.66 Bohr units, lies 26.7 kcal/mole lower than Li2 + H2 and 41.3 kcal/mole lower than 2 LiH. Fundamental vibrational frequencies and certain characteristics of potential surfaces are presented for both stable species. The properties of four unstable linear dimers are examined.