Static Ab Initio Calculations Research Articles

Insights Into Curie‐Temperature and Phase Formation of Ferroelectric Hf1−xZrxO2 with Oxygen Defects from a Leveled Energy Landscape

AbstractThe phase composition of HZO thin films is critical for the ferroelectric and electrical properties of the films and the devices they are integrated into. Optimization is a major challenge since the phase formation depends significantly on many influencing variables that are only partially understood so far. The Curie temperature is identified as an important parameter for understanding the behavior, since it depends sensitively on Zr content, the density of oxygen‐related defects, layer thickness, and external stress. A two‐step process, phase formation by pure kinetic transformation followed by nucleation, is proposed for phase formation. This is necessary because nucleation theory alone cannot explain the experimentally observed dependence on oxygen content. The classical nucleation model is modified at two crucial points. First, the polycrystalline structure is incorporated which allows the size effect to be implemented. Furthermore, the interface energies between the child and parent phase, which result from static ab initio calculations, are rescaled from dynamical effects. The resulting model is used to calculate the phase fractions during thermal processing. The results for the most important influencing variables are discussed and compared with experimental results. The causes of the undesired monoclinic phase are further analyzed.

C-C Bond Formation Reaction Catalyzed by a Lithium Atom: Benzene-to-Biphenyl Coupling.

Transition-metal-catalyzed carbon-carbon (C-C) bond formation is an important reaction in pharmaceutical and organic chemistry. However, the reaction process is composed of multiple steps and is expensive owing to the presence of transition metals. This study proposes a lithium-catalyzed C-C coupling reaction of two benzene molecules (Bz) to form a biphenyl molecule, which is a transition-metal-free reaction, based on ab initio and direct ab initio molecular dynamics (AIMD) calculations. The static ab initio calculations indicate that the reaction of two Bz molecules with Li- ions (reactant state, RC) can form a stable sandwiched complex (precomplex), where the Li- ion is sandwiched by two Bz molecules. The complex formation reaction can be expressed as 2Bz + Li - → Bz(Li -)Bz, where the C-C distance between the Bz rings is 2.449 Å. This complex moves to the transition state (TS) via the structural deformation of Bz(Li-)Bz, where the C-C distance is shortened to 2.118 Å. The barrier height was calculated to be -9.9 kcal/mol (relative to RC) at the MP2/6-311++G(d,p) level. After TS, the C(sp3)-C(sp3) single bond was completely formed between the Bz rings (the C-C bond distance was 1.635 Å) (late complex). After the dissociation of H2 from the late complex, a biphenyl molecule was formed: the C(sp2)-C(sp2) bond. The calculations suggest that the C-C bond coupling of Bz occurred spontaneously from 2Bz + Li-, and biphenyl molecules were directly formed without an activation barrier. Direct AIMD calculations show that the C-C coupling reaction also takes place under electron attachment to Li(Bz)2: Li(Bz)2 + e- → [Li-(Bz)2]ver → precomplex → TS → late complex, where [Li-(Bz)2]ver is the vertical electron capture species of Li(Bz)2. Namely, the C-C coupling reaction spontaneously occurred in Li(Bz)2 owing to electron attachment. Similar C-C coupling reactions were also observed for halogen-substituted benzene molecules (Bz-X, X = F and Cl). Furthermore, this study discusses the mechanism of C-C bond formation in electron capture based on the theoretical results.

Formation Mechanism of Odd- and Even-Numbered Hydrogen Cluster Cations Using the Direct Ab Initio Molecular Dynamics Approach.

The photoreaction of hydrogen clusters caused by cosmic-ray irradiations plays a crucial role in interstellar molecular clouds because the reaction of molecules takes place in these surrounding clusters. Previous gas-phase experiments introduced the reaction products after the ionization of the neutral hydrogen cluster. In the experiments, the odd-numbered hydrogen cluster cation, H+(H2)m, was the main product, while the even-numbered cluster cation, (H2)n+, was the minor product. However, the formation yield of (H2)n+ was significantly low. Despite many theoretical calculations and experiments, the formation mechanism of odd- and even-numbered cluster ions from ionized hydrogen clusters is still unclear. In this study, the direct ab initio molecular dynamics (AIMD) method was applied to the reaction of the hydrogen cluster cation (H2)n+ to understand the abovementioned formation mechanism. The trajectories of (H2)n+ following the vertical ionization of the neutral cluster were calculated. Direct AIMD calculations indicated that odd-numbered cluster cations, H3+(H2)n-2 + H (dissociation), were formed directly as the main product after the ionization of (H2)n. An even-numbered cluster, (H2)n+, was temporally formed when an H-shaped complex composed of three H2 molecules existed within a neutral cluster. However, it was rapidly dissociated to an odd-numbered cluster. Static ab initio calculations suggested that the odd-numbered cluster complex, H3+(H2)n-2-H, could be transferred to even-numbered ions via a transition state with a low energy barrier. The even-numbered cluster cation was formed stepwise from the odd-numbered cluster cation. The formation mechanism of odd- and even-numbered cluster cations is discussed based on these results.

Jahn-Teller Effect of the Benzene Radical Cation: A Direct ab Initio Molecular Dynamics Study.

The benzene radical cation (Bz+) is a typical model molecule of the Jahn-Teller (J-T) active species. Bz+ has two structural forms due to the J-T effect. These are the compressed and elongated forms, expressed as Bz+(comp) and Bz+(elong), respectively. In Bz+(comp), the hexagonal structure of the benzene ring is compressed up and down, and in Bz+(elong), it is pulled up and down. From electron spin resonance experiments, it was found that Bz+ takes a compressed form in low-temperature Freon matrices (CF3Cl and CF2ClCFCl2), whereas the elongated form was found in argon matrices. However, the selectivity of these structural forms is still unclear. In this study, the ionization dynamics of isolated benzene (Bz) and benzene-M complexes (where M denotes counter-molecules, M = NH3, H2O, CF3Cl, CH4, CH3OH, Ar, SH2, ammonia dimer, or water dimer) have been investigated by means of the direct ab initio molecular dynamics (AIMD) method in order to shed light on the Bz+ formation mechanism. The static ab initio calculations showed that Bz+(comp) is slightly more energetically stable than Bz+(elong), although the energy difference was only 0.1 kcal/mol at the CCSD/6-311++G(d,p) level. The direct AIMD calculations indicated that Bz+(comp) was formed from the Bz-M complexes when M was NH3, CF3Cl, or an ammonia dimer, whereas the ionization of Bz-M when M was H2O, CH4, CH3OH, SH2, or a water dimer formed Bz+(elong). In the case of complexes with an argon dimer, Bz(Ar)2, both forms were obtained from a slight orientation change of Ar on Bz. A selective rule is discussed on the basis of the calculated results.

Ab initio study of hydrogen on beryllium surfaces

Static ab initio calculations were performed for five principal hexagonal close-packed beryllium surfaces: basal, prismatic (type I and II) and pyramidal (type I and II). The basal plane was found to be the most energetically favorable, while the energies of the prismatic (type I) and pyramidal (type I) planes were slightly higher followed by the type II planes. Beryllium is known to show extreme interlayer distance relaxation near the surface. Up to five outermost atomic layers were involved in surface relaxation. The presence of hydrogen on the beryllium surfaces led to a noticeable reduction of the surface energy.

Excitonic splittings in molecular dimers: why static ab initio calculations cannot match them.

After decades of research on molecular excitons, only few molecular dimers are available on which exciton and vibronic coupling theories can be rigorously tested. In centrosymmetric H-bonded dimers consisting of identical (hetero)aromatic chromophores, the monomer electronic transition dipole moment vectors subtract or add, yielding S0 → S1 and S0 → S2 transitions that are symmetry-forbidden or -allowed, respectively. Symmetry breaking by 12C/13C or H/D isotopic substitution renders the forbidden transition weakly allowed. The excitonic coupling (Davydov splitting) can then be measured between the S0 → S1 and S0 → S2 vibrationless bands. We discuss the mass-specific excitonic spectra of five H-bonded dimers that are supersonically cooled to a few K and investigated using two-color resonant two-photon ionization spectroscopy. The excitonic splittings Δcalc predicted by ab initio methods are 5-25 times larger than the experimental excitonic splittings Δexp. The purely electronic ab initio splittings need to be reduced ("quenched"), reflecting the coupling of the electronic transition to the optically active vibrations of the monomers. The so-called quenching factors Γ < 1 can be determined from experiment (Γexp) and/or calculation (Γcalc). The vibronically quenched splittings Γ·Δcalc are found to nicely reproduce the experimental exciton splittings.

Ti and Zr Transition Metals Effect in the D03 Fe3Al by &lt;i&gt;Ab Initio&lt;/i&gt; Study Approach

The effect of the Ti and Zr transition metals on the D03-Fe3Al intermetallic compounds has been investigated by means of ab initio Pseudo Potentials numerical simulations based on Density Functional Theory. Two main issues will be addressed the understanding of the role of these two transition metals in terms of stability of the bulk at the light of their site preference in the D03-Fe3Al structure the behaviour of Ti and Zr transition metals in the sigma 5 (310) [001] grain boundary and their effect on the structural stability of this interface. An important issue when studying these aspects is to take into accounts the effect of temperature. This requires a molecular dynamics treatment of the atoms in the supercell. The technique known as ab initio molecular dynamics (AIMD) solves these problems by combining ‘on the fly’ electronic structure calculations with finite temperature dynamics. Thus, our study was conducted both using the conventional static ab initio calculations (0K) as well as by taking into account the effect of temperature (Ab Initio Molecular Dynamics).

Ab initio static and molecular dynamics studies of helium behavior in beryllium

Beryllium is an effective neutron multiplier material widely exploited in nuclear applications. It will be used in the helium-cooled beryllium pebble bed of fusion reactor blankets for increasing the efficiency of tritium production. Macroscopic effects of irradiation (e.g., swelling) on beryllium are greatly influenced by accumulated transmutation helium. Atomic scale simulations of beryllium behavior under irradiation are necessary for understanding the basic mechanisms and reliable prediction of microstructural changes.In this study, we investigate the behavior of interstitial and substitutional helium, its diffusion pathways and interaction with point defects present in irradiated beryllium by means of static and molecular dynamics ab initio simulations. It was shown that a mixed dumbbell consisting of self-interstitial and helium atoms represents the ground-state configuration for interstitial helium. At low temperatures, the mixed dumbbell migrates in the basal plane through a series of in-basal-plane rotations, while at higher temperatures it jumps also between adjacent basal planes.It was revealed that, as in many other metals, interstitial helium atoms are bound to each other (Eb∼1eV). In beryllium, two mixed dumbbells meet each other so that helium atoms are the nearest neighbors, whereas the helium pair can be oriented either in- or out-of-basal plane. Diffusion modes of the pair are discussed.In addition, it was found that helium is very strongly bound to vacancies: the binding energy of more than 3eV found by static ab initio calculations suggests that helium is unlikely to be released from a mono-vacancy at temperatures below the melting point. As has been shown previously, vacancy clusters are unstable in beryllium. This study shows that the addition of helium stabilizes di-vacancies. Thus, it is confirmed that the presence of transmutation gas is necessary for the development of a porous microstructure in beryllium.

Nonadiabatic dynamics of a truncated indigo model

Indigo (1) is stable when exposed to ultraviolet light. We employ electronic structure calculations and nonadiabatic trajectory surface-hopping dynamics simulations to study the photoinduced processes and the photoprotection mechanism of an indigo model, bispyrroleindigo (2). Consistent with recent static ab initio calculations on 1 and 2 (Phys. Chem. Chem. Phys., 2011, 13, 1618), we find an efficient deactivation process that proceeds as follows. After vertical photoexcitation, the S(1)(ππ*) state undergoes an essentially barrierless intramolecular single proton transfer and relaxes to the minimum of an S(1) tautomer, which is structurally and energetically close to a nearby conical intersection that acts as a funnel to the S(0) state; after this internal conversion, a reverse single hydrogen transfer leads back to the equilibrium structure of the most stable S(0) tautomer. This deactivation process is completely dominant in our semiempirical OM2/MRCI nonadiabatic dynamics simulations. The other two mechanisms considered previously, namely excited-state intramolecular double proton transfer and trans-cis double bond isomerization, are not seen in any of the 325 trajectories of the present surface-hopping simulations. On the basis of the computed time-dependent populations of the S(1) state, we estimate an S(1) lifetime of about 700 fs for 2 in the gas phase.

Ab initio theoretical calculations of the electronic excitation energies of small water clusters

A direct ab initio molecular dynamics method has been applied to a water monomer and water clusters (H(2)O)(n) (n = 1-3) to elucidate the effects of zero-point energy (ZPE) vibration on the absorption spectra of water clusters. Static ab initio calculations without ZPE showed that the first electronic transitions of (H(2)O)(n), (1)B(1)←(1)A(1), are blue-shifted as a function of cluster size (n): 7.38 eV (n = 1), 7.58 eV (n = 2) and 8.01 eV (n = 3). The inclusion of the ZPE vibration strongly affects the excitation energies of a water dimer, and a long red-tail appears in the range of 6.42-6.90 eV due to the structural flexibility of a water dimer. The ultraviolet photodissociation of water clusters and water ice surfaces is relevant to these results.

Formamide Dimers: A Computational and Matrix Isolation Study

The dimerization of formamide (FMA) has been investigated by matrix isolation spectroscopy, static ab initio calculations, and ab initio molecular dynamics (AIMD) simulations. Comparison of the experimental matrix IR spectra with the ab initio calculations reveals that two types of dimers A and C are predominantly formed, with two and one strong NH...O hydrogen bonds, respectively. This is in accordance with previously published experiments. In addition, there is also experimental evidence for the formation of the thermally labile dimer B after deposition of high concentrations of FMA in solid xenon. The AIMD simulations of the aggregation process show that in all cases dimer C is initially formed, but rearrangement to the more stable doubly hydrogen-bonded structures A or B occurs for a fraction of collisions on the sub-picosecond time scale.

Intramolecular SN2 Reaction Caused by Photoionization of Benzene Chloride−NH3 Complex: Direct ab Initio Molecular Dynamics Study

Ionization processes of chlorobenzene-ammonia 1:1 complex (PhCl-NH3) have been investigated by means of full dimensional direct ab initio molecular dynamics (MD) method, static ab initio calculations, and density functional theory (DFT) calculations. The static ab initio and DFT calculations of neutral PhCl-NH3 complex showed that one of the hydrogen atoms of NH3 orients toward a carbon atom in the para-position of PhCl. The dynamics calculation for ionization of PhCl-NH3 indicated that two reaction channels are competitive with each other as product channels: one is an intramolecular SN2 reaction expressed by a reaction scheme [PhCl-NH3]+-->SN2 intermediate complex-->PhNH3++Cl, and the other is ortho-NH3 addition complex (ortho complex) in which NH3 attacks the ortho-carbon of PhCl+ and the trajectory leads to a bound complex expressed by (PhCl-NH3)+. The mechanism of the ionization of PhCl-NH3 is discussed on the basis of the theoretical results.

Ionization Dynamics of trans-Formanilide−H2O Complexes: A Direct ab Initio Dynamics Study

Ionization dynamics of trans-formanilide-water 1:1 complexes FA(H2O) have been investigated by means of direct ab initio trajectory method. From the static ab initio calculations, three conformers of the FA(H2O) complexes were obtained as stable structures: namely, these are the N−H site, the CO carbonyl site and the bridge site, which are different in the positions of H2O around FA. In the N−H and CO sites, a water molecule binds to the hydrogen and oxygen atoms of the peptide (−NH−CO), respectively. In the bridge site, the hydrogen and oxygen atoms of H2O bind to the CO carbonyl and a hydrogen of benzene ring (ο-position) of FA, respectively. The trajectories from the vertical ionization points of these three structures were calculated by means of full dimensional direct ab initio trajectory method. It was found that the H2O molecule in the N−H site is still remained in its site after ionization, i.e., the strong complex cation where H2O binds to the N−H site of FA+ is directly formed. On the other hand, in the cases of the ionization from both CO and bridge sites, the water molecule was moved easily around both the benzene ring and the CO carbonyl group. The mechanism of the ionization of FA(H2O) was discussed on theoretical results.

Ab initio and experimental study on thermally degradable polycarbonates: Effect of structure on reactivity

AbstractThe thermal degradation process of some new synthesized polycarbonates is studied both from theoretical and experimental points of view to obtain microscopic insight into the factors that govern the reaction rates. Ab initio density functional theory calculations are performed on a set of carbonates, which are model systems for the polycarbonates. Kinetic parameters are determined by transition‐state theory and compared with experimental‐derived degradation temperatures. The study as presented here gives evidence of the existence of a correlation between these degradation temperatures and the reaction rates resulting from static ab initio calculations on small model systems that are representative for the active area of the degradation process. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

A Direct ab Initio Trajectory Study on the Gas-Phase SN2 Reaction OH- + CH3Cl → CH3OH + Cl-

Direct ab initio trajectory calculations have been applied to a SN2 reaction, OH- + CH3Cl → CH3OH + Cl-. First, static ab initio molecular orbital (MO) calculations with several basis sets were examined to select the most convenient and best fit basis set to that of high-quality calculations. As a result of the static ab initio calculations, it was found that the Hartree−Fock (HF)/3-21+G(d) calculation reasonably represents a potential energy surface calculated at the MP2/6-311++G(2df,2pd) level. Next, direct ab initio dynamics calculations using the 3-21+G(d) basis set were carried out for the SN2 reaction. A full dimensional potential energy surface including all degrees of freedom was used in the dynamics calculation. The collision energies chosen were Ecoll = 5 and 25 kcal/mol. In the collisions at Ecoll = 5 and 25 kcal/mol, 48% and 63% of the total available energies were, respectively, partitioned into the relative translational mode between CH3OH and Cl-. Also, it was predicted that the C−H stretching mode of the product CH3OH is excited after the SN2 reaction, which is not detected in the case of the halogen-atom exchange SN2 reaction. The reaction mechanism was discussed on the basis of theoretical results.

Thermodynamic Stability of Fe/O Solid Solution at Inner‐Core Conditions

We present a new technique which allows the fully ab initio calculation of the chemical potential of a substitutional impurity in a high‐temperature crystal, including harmonic and anharmonic lattice vibrations. The technique uses the combination of thermodynamic integration and reference models developed recently for the ab initio calculation of the free energy of liquids and anharmonic solids. We apply the technique to the case of the substitutional oxygen impurity in h.c.p. iron under Earth's core conditions, which earlier static ab initio calculations indicated to be thermodynamically very unstable. Our results show that entropic effects arising from the large vibrational amplitude of the oxygen impurity give a major reduction of the oxygen chemical potential, so that oxygen dissolved in h.c.p. iron may be stabilised at concentrations up a few mol % under core conditions.

The Three Isomers of Protonated Ethane, C2H7+

The fluxional behavior of the protonated ethane ion was examined using both static and dynamic modeling. Static ab initio calculations, including perturbation theory (MP2), coupled cluster (CCSD(T)), and density functional theory, were used to locate various minima, saddle points, and G2-quality relative energies on the potential energy surface for atomic motions. In tandem, Car−Parrinello molecular dynamics simulations were performed to aid the stationary-point search and to examine the stabilities of various isomers at different temperatures. Predicted infrared spectra were also obtained from both techniques. Unlike most previous experimental and theoretical investigations which have focused upon the relative energies and stabilities of σC-C-protonated (bridged) structures and σC-H-protonated (open or “classical”) structures, this work establishes the existence of a third isomer, the ion−molecule or solvated-ion complex C2H5+···H2, which is the more likely candidate for the second isomer of experiments by Hiraoka and Kebarle and by Yeh, Price, and Lee. The open isomer may still be experimentally unknown. Peculiar discrepancies remain, however, and further experimental work is needed to resolve them.