Efficient evaluation of the accuracy of molecular quantum dynamics on an approximate analytical or interpolated ab initio potential energy surface

Ab initio electronic structure methods have reached a satisfactory accuracy for the calculation of static properties, but remain too expensive for quantum dynamical calculations. Recently, an efficient semiclassical method was proposed to evaluate the accuracy of quantum dynamics on an approximate potential without having to perform the expensive quantum dynamics on the accurate potential. Here, this method is applied for the first time to evaluate the accuracy of quantum dynamics on an approximate analytical or interpolated potential in comparison to the quantum dynamics on an accurate potential obtained by an ab initio electronic structure method. Specifically, the vibrational dynamics of H2 on a Morse potential is compared with that on the full CI potential, and the photodissociation dynamics of CO2 on a LEPS potential with that on the excited 1^Pi surface computed at the EOM-CCSD/aug-cc-pVDZ level of theory. Finally, the effect of discretization of a potential energy surface on the quantum dynamics is evaluated.

Efficient evaluation of accuracy of molecular quantum dynamics using dephasing representation

Cesare Mollica, Jiri Vanicek

Ab initio methods for the electronic structure of molecules have reached a satisfactory accuracy for calculations of static properties but remain too expensive for quantum dynamics calculations. We propose an efficient semiclassical method for evaluating the accuracy of a lower level quantum dynamics, as compared to a higher level quantum dynamics, without having to perform any quantum dynamics. The method is based on the dephasing representation of quantum fidelity and its feasibility is demonstrated on the photodissociation dynamics of CO2. Our accuracy test can be easily implemented in existing molecular dynamics codes, thus offering wide applicability.

American Institute of Physics2009

Molecular Dynamics Simulations of Helium Atoms Clustering in bcc Iron

Ning Gao

Helium atoms are known to have a significant impact on materials used in fission and fusion reactors. In particular, the presence of helium atoms can change the mechanical properties and degrade the lifetime of reactors. In order to develop the helium-resistance materials, the underlying interactions between helium atoms and matrix atoms have to be understood. In the past, atomistic simulations have contributed a lot to the basic energy states of helium atoms in different defects of materials. Especially, insights into the formation energies of helium atoms at simple defects have been calculated by ab initio and molecular dynamics. The substitional helium has lower formation energy than the tetrahedral and octahedral interstitial helium atom. Additionally, the binding energies of helium with small HenVm cluster have also been found to be all positive. However, all these simulations have been performed without showing the kinetic formation process of such helium defects. The interactions of defect with larger number of helium atoms are also not explored in detail. In the first part of this thesis, a new relaxation-fitting method has been developed for cross potential of Fe-He system based on more energy data of helium-defect clusters from ab initio calculations and by including the migration energy of single free helium atom in the fitting process. Besides, the recently fitted magnetic potentials of Fe have been used to describe the Fe-Fe interactions for such cross potential. The new fitted Fe-He potentials have predicted well not only the properties of simple helium defect, but also the properties of helium clusters and diffusion process of helium atoms. Based on the new Fe-He potentials, the interactions between helium atoms and Fe with the effect of single vacancy have been simulated utilizing the classical molecular dynamics (MD) method in the single bcc Fe lattice at room temperatures by helium atoms up to 18. The binding energies of helium atom with HenV cluster are computed. By increasing number of helium atoms, the binding energy decreases first, then slowly increases up to n = 4. It is then almost constant between n = 5 to 15 but increases strongly at n = 16 to form a peak. This last increase is related to the athermal self-interstitial atom (SIA) emission in the form of dumbbell which relaxes the restrained system to He16V2 cluster. The second binding energy peak occurs at n = 18 by the formation of the second SIA to form He18V3 cluster. Close inspection of the local configuration shows the formed SIA combines with the helium-defect cluster. Calculation of the pressure of the helium-defect cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of helium-defect cluster suggest that with increasing helium content, some symmetrical structures can be formed. By increasing the number of helium atoms inserted in single bcc Fe lattice, the clustering process of the formed SIAs with presence of helium cluster has been simulated with MD methods at room temperatures with the new fitted Fe-He potential. The first SIA is formed after 6 He atoms being inserted without vacancy. The simulations indicate that the SIA has mainly two configurations as either a dumbbell or a crowdion, which can be exchanged in the interface between helium bubble and matrix atoms. The SIA cluster can change direction and absorb more SIAs to grow. Small dislocation loops have been observed to form with Burgers vector b = 1/2 a0 on a {111} habit plane. The kinetic process of formation and growth of dislocation loop explored by MD simulations includes: (1) the formation of a self-interstitial atom by the high pressure of the helium cluster/bubble; (2) the diffusion of these self-interstitial atoms on the interface between helium bubble and matrix; (3) the clustering of the self-interstitial atoms in the form of a dislocation loop and its diffusion away from bubble when the number of SIAs in loop reaches a critical number. Moreover, the effect of grain boundary (GB) on the formation of helium clusters has been explored with MD simulations in symmetrical GBs and normal nano-grain boundary samples. Helium atoms can be trapped by GBs at free spaces. The local atomic excess free volume (LAEFV) is found to the one of the main parameters not only related with the energy state of GB but also with the nucleation and growth of He atoms in the GB.

EPFL2011

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.