136 research outputs found
Electronic polarization in quasilinear chains
Starting with a finite -mesh version of a well-known equation by Blount,
we show how various definitions proposed for the polarization of long chains
are related. Expressions used for infinite periodic chains in the 'modern
theory of polarization' are thereby obtained along with a new single particle
formulation. Separate intracellular and intercellular contributions to the
polarization are identified and, in application to infinite chains, the
traditional sawtooth definition is found to be missing the latter. For a finite
open chain the dipole moment depends upon how the chain is terminated, but the
intracellular and intercellular polarization do not. All of these results are
illustrated through calculations with a simple H\"uckel-like model.Comment: 5 page
First-Principles Calculation of the Optical Rotatory Power of Periodic Systems: Modern Theory with Modern Functionals
An analysis of orbital magnetization in band insulators is provided. It is
shown that a previously proposed electronic orbital angular-momentum operator
generalizes the ``modern theory of orbital magnetization'' to include non-local
Hamiltonians. Expressions for magnetic transition dipole moments needed for the
calculation of optical rotation (OR) and other properties are developed. A
variety of issues that arise in this context are critically analyzed. These
issues include periodicity of the operators, previously proposed band
dispersion terms as well as, if and where needed, evaluation of reciprocal
space derivatives of orbital coefficients. Our treatment is used to determine
the optical rotatory power of band insulators employing a formulation that
accounts for electric dipole - electric quadrupole (DQ), as well as electric
dipole-magnetic dipole, contributions. An implementation in the public
\textsc{Crystal} program is validated against a model finite system and applied
to the -quartz mineral through linear-response time-dependent density
functional theory with a hybrid functional. The latter calculations confirmed
the importance of DQ terms. Agreement against experiment was only possible with
i) use of a high quality basis set, ii) inclusion of a fraction of non-local
Fock exchange, and iii) account of orbital-relaxation terms in the calculation
of response functions
Factors Governing Nuclear Geometry and Bond-Orbital Directions in Second Row AH2 Molecules
We have obtained valence bond waivefunctions for the neutral,
positive ion, and lower excited states of second row AH2 molecules.
A number of simple rules emerge which govern the nuclear geometry
and bond-orbital directions. These are explained in terms of valence bond orbital characteristics as well as an analysis of energy components
How much can donor/acceptor-substitution change the responses of long push-pull systems to DC fields?
Mathematical arguments are presented that give a unique answer to the
question in the title. Subsequently, the mathematical analysis is extended
using results of detailed model calculations that, in addition, throw further
light on the consequences of the analysis. Finally, through a comparison with
various recent studies, many of the latter are given a new interpretation.Comment: Accepted by Chem. Phys. Let
Ab initio analytical Raman intensities for periodic systems through a coupled perturbed Hartree-Fock/Kohn-Sham method in an atomic orbital basis. I. Theory
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
Anharmonic vibrational analysis of water with traditional and explicitly correlated coupled cluster methods
It is well known that the convergence of harmonic frequencies with respect to the basis set size in traditional correlated calculations is slow. We now report that the convergence of cubic and quartic force constants in traditional CCSD(T) calculations on H(2)O with Dunning's cc-pVXZ family of basis sets is also frustratingly slow. As an alternative, we explore the performance of R12-based explicitly correlated methods at the CCSD(T) level. Excellent convergence of harmonic frequencies and cubic force constants is provided by these explicitly correlated methods with R12-suited basis irrespective of the used standard approximation and/or the correlation factor. The Slater type geminal, however, outperforms the linear r(12) for quartic force constants and vibrational anharmonicity constants. The converged force constants from explicitly correlated CCSD(T) calculations succeed in reproducing the fundamental frequencies of water molecule with spectroscopic accuracy after corrections for post-CCSD(T) effects are made
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