834 research outputs found
Exploitation of symmetry in periodic Self-Consistent-Field ab initio calculations: application to large three-dimensional compounds
Symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for crystalline systems. Crucial for running time is use of symmetry in the evaluation of one- and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. We here illustrate the effectiveness of this scheme, following recent advancements in the CRYSTAL code, concerning memory allocation and direct basis set transformation. Quantitative examples are given for large unit cell systems, such as zeolites (all-silica faujasite and silicalite MFI) and garnets (pyrope). It is shown that the full SCF of 3D systems containing up to 576 atoms and 11136 Atomic Orbitals in the cell can be run with a hybrid functional on a single core PC with 500 MB RAM in about 8 h. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg
Communication: Hole localization in Al-doped quartz SiO2 within ab initio hybrid-functional DFT
We investigate the long-standing problem of the hole localization at the Al
impurity in quartz SiO, using a relatively recent DFT hybrid-functional
method in which the exchange fraction is obtained \emph{ab initio}, based on an
analogy with the static many-body COHSEX approximation to the electron
self-energy. As the amount of the admixed exact exchange in hybrid functionals
has been shown to be determinant for properly capturing the hole localization,
this problem constitutes a prototypical benchmark for the accuracy of the
method, allowing one to assess to what extent self-interaction effects are
avoided. We obtain good results in terms of description of the charge
localization and structural distortion around the Al center, improving with
respect to the more popular B3LYP hybrid-functional approach. We also discuss
the accuracy of computed hyperfine parameters, by comparison with previous
calculations based on other self-interaction-free methods, as well as
experimental values. We discuss and rationalize the limitations of our approach
in computing defect-related excitation energies in low-dielectric-constant
insulators.Comment: Accepted for publication in J. Chem. Phys. (Communications
5D quantum dynamics of the H2@SWNT system: Quantitative study of the rotational-translational coupling
The dynamics of the dihydrogen molecule when confined in carbon nanotubes with different chiral- ities and diameters are studied by using a 5 dimensional model considering the most relevant degrees of freedom of the system. The nuclear eigenstates are calculated for an (8,0) and a (5,0) carbon nanotubes by the State-Average Multiconfigurational Time-dependent Hartree, and then studied using qualitative tools (mapping of the total wave functions onto given subspaces) and more rigorous analysis (different kinds of overlaps with reference functions). The qualitative analysis is seen to fail due to a strong coupling between the internal and translational degrees of freedom. Using more accurate tools allows us to gain a deeper insight into the behaviour of confined species
A new structural model for disorder in vaterite from first-principles calculations
Both of the previously proposed Pbnm and P6522 ordered structures for vaterite are found to be unstable transition states using first principles methods. Five stable structures are located, the lowest energy one being of P3221 symmetry. Since interconversion between these structures requires only thermal energy, this provides an additional source of disorder within the vaterite structure
Assessing Thermochemical Properties of Materials through Ab initio Quantum-mechanical Methods: The Case of α-Al2O3
The thermochemical behavior of α-Al2O3 corundum in the whole temperature range 0–2317 K (melting point) and under pressures up to 12 GPa is predicted by applying ab initio methods based on the density functional theory (DFT), the use of a local basis set and periodic-boundary conditions. Thermodynamic properties are treated both within and beyond the harmonic approximation to the lattice potential. In particular, a recent implementation of the quasi-harmonic approximation, in the Crystal program, is here shown to provide a reliable description of the thermal expansion coefficient, entropy, constant-volume and constant-pressure specific heats, and temperature dependence of the bulk modulus, nearly up to the corundum melting temperature. This is a remarkable outcome suggesting α-Al2O3 to be an almost perfect quasi-harmonic crystal. The effect of using different computational parameters and DFT functionals belonging to different levels of approximations on the accuracy of the thermal properties is tested, providing a reference for further studies involving alumina polymorphs and, more generally, quasi-ionic minerals
Probing vibrational modes in silica glass using inelastic neutron scattering with mass contrast
The effective vibrational density of states (VDOS) has been derived from inelastic neutron-scattering data, for isotopically substituted Si O 18 2 and Si O 16 2 glasses, to gain information about the relative contribution to the Si and O partial VDOS. This is a necessary point of comparison for vibrational mode analyses of molecular-dynamics models. The mass contrast has led to a measurable shift between vibrational mode frequencies in the effective VDOS of Si O 18 2 and Si O 16 2, which is well reproduced in an ab initio simulation. The vibrational band centered at 100.2 meV is confirmed to have significantly lower contribution from the oxygen partial VDOS, than the higher (150.3 and 135.8 meV) and lower energy bands (53.3 meV)
Bulk and surface energetics of lithium hydride crystal: benchmarks from quantum Monte Carlo and quantum chemistry
We show how accurate benchmark values of the surface formation energy of
crystalline lithium hydride can be computed by the complementary techniques of
quantum Monte Carlo (QMC) and wavefunction-based molecular quantum chemistry.
To demonstrate the high accuracy of the QMC techniques, we present a detailed
study of the energetics of the bulk LiH crystal, using both pseudopotential and
all-electron approaches. We show that the equilibrium lattice parameter agrees
with experiment to within 0.03 %, which is around the experimental uncertainty,
and the cohesive energy agrees to within around 10 meV per formula unit. QMC in
periodic slab geometry is used to compute the formation energy of the LiH (001)
surface, and we show that the value can be accurately converged with respect to
slab thickness and other technical parameters. The quantum chemistry
calculations build on the recently developed hierarchical scheme for computing
the correlation energy of a crystal to high precision. We show that the
hierarchical scheme allows the accurate calculation of the surface formation
energy, and we present results that are well converged with respect to basis
set and with respect to the level of correlation treatment. The QMC and
hierarchical results for the surface formation energy agree to within about 1
%.Comment: 16 pages, 4 figure
Calculation of the Infrared Intensity of Crystalline Systems. A Comparison of Three Strategies Based on Berry Phase, Wannier Function, and Coupled-Perturbed Kohn–Sham Methods
Three
alternative strategies for the calculation of the IR intensity
of crystalline systems, as determined by Born charges, have been implemented
in the Crystal code, using a Gaussian type basis set. One
uses the Berry phase (BP) algorithm to compute the dipole moment;
another does so, instead, through well localized crystalline orbitals
(Wannier functions, WF); and the third is based on a coupled perturbed
Hartree–Fock or Kohn–Sham procedure (CP). In WF and
BP, the derivative of the dipole moment with respect to the atomic
coordinates is evaluated numerically, whereas in CP it is analytical.
In the three cases, very different numerical schemes are utilized,
so that the equivalence of the obtained IR intensities is not ensured
a priori but instead is the result of the high numerical accuracy
of the many computational steps involved. The main aspects of the
three schemes are briefly recalled, and the dependence of the results
on the computational parameters (number of k points
in reciprocal space, tolerances for the truncation of the Coulomb
and exchange series, and so on) is documented. It is shown that in
standard computational conditions the three schemes produce IR intensities
that differ by less than 1%; this difference can be reduced by an
order of magnitude by acting on the parameters that control the accuracy
of the calculation. A large unit cell system (80 atoms per cell) is
used to document the relative cost of the three schemes. Within the
current implementation the BP strategy, despite its seminumerical
nature, is the most efficient choice. That is because it is the oldest
implementation, and it is based on the simplest of the three algorithms.
Thus, parallelism and other schemes for improving efficiency have,
so far, been implemented to a lesser degree in the other two cases
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