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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.

CRYSTAL is a periodic ab initio code that uses a Gaussian-type basis set to express crystalline orbitals (i.e., Bloch functions). The use of atom-centered basis functions allows treating 3D (crystals), 2D (slabs), 1D (polymers), and 0D (molecules) systems on the same grounds. In turn, all-electron calculations are inherently permitted along with pseudopotential strategies. A variety of density functionals are implemented, including global and range-separated hybrids of various natures and, as an extreme case, Hartree–Fock (HF). The cost for HF or hybrids is only about 3–5 times higher than when using the local density approximation or the generalized gradient approximation. Symmetry is fully exploited at all steps of the calculation. Many tools are available to modify the structure as given in input and simplify the construction of complicated objects, such as slabs, nanotubes, molecules, and clusters. Many tensorial properties can be evaluated by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, first and second hyperpolarizabilities, etc. The calculation of infrared and Raman spectra is available, and the intensities are computed analytically. Automated tools are available for the generation of the relevant configurations of solid solutions and/or disordered systems. Three versions of the code exist: serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated on each core, whereas in the third one, the Fock matrix is distributed for diagonalization. All the relevant vectors are dynamically allocated and deallocated after use, making the code very agile. CRYSTAL can be used efficiently on high performance computing machines up to thousands of cores. [ABSTRACT FROM AUTHOR]

The response to electrostatic fields by finite systems, which contain a large central region of chemically identical building blocks, is shown not to depend on the boundaries of the system. Consequently, we can determine this response from that of the corresponding infinite periodic system and a vector potential theoretical-computational approach is presented for doing so. Both the electronic and structural response can be obtained with our approach. In many cases, neglecting the structural response is a poor approximation. To illustrate the performance of our treatment, extensive calculations have been carried out for simple model systems. Selected results are reported. Key words: polarization, extended systems, electrostatic fields, structural properties, electronic properties. On demontre que, pour des systemes finis contenant une grande region de blocs chimiquement identiques, la reponse a des champs electrostatiques ne depend pas des limites du systeme. En consequence, on peut determiner cette reponse a partir des systemes infinis correspondants et on presente une approche par calculs theoriques des potentiels vectoriels qui permet d'y arriver. Avec notre approche, il est possible d'obtenir aussi bien la reponse electronique que structurelle. Dans plusieurs cas, si on neglige la reponse structurelle, on obtient une mauvaise approximation. Afin d'illustrer la performance de notre traitement, on a effectue des calculs extensifs de systemes modeles simples. On rapporte des resultats choisis. Mots-cles : polarisation, systemes etendus, champs electrostatiques, proprietes structurelles, proprietes electroniques., [Traduit par la Redaction] Introduction With this contribution, we want to honor Tom Ziegler on the occasion of his 65th birthday. It happens that Tom Ziegler shares a number of [...]

Atheoretical treatment for the orbital response of an infinite, periodic system to a static, homogeneous, magnetic field is presented. It is assumed that the system of interest has an energy gap separating occupied and unoccupied orbitals and a zero Chern number. In contrast to earlier studies, we do not utilize a perturbation expansion, although we do assume the field is sufficiently weak that the occurrence of Landau levels can be ignored. The theory is developed by analyzing results for large, finite systems and also by comparing with the analogous treatment of an electrostatic field. The resulting many-electron Hamilton operator is forced to be hermitian, but hermiticity is not preserved, in general, for the subsequently derived single-particle operators that determine the electronic orbitals. However, we demonstrate that when focusing on the canonical solutions to the single-particle equations, hermiticity is preserved. The issue of gauge-origin dependence of approximate solutions is addressed. Our approach is compared with several previously proposed treatments, whereby limitations in some of the latter are identified. [ABSTRACT FROM AUTHOR]

Research into the treatment of dipole moments, polarizabilities and hyperpolarizabilities is presented.

Density functional theory schemes based on conventional exchange-correlation functionals have been used to establish dipole moment, polarizability, and first and second hyperpolarizabilities of push-pull pi-conjugated systems. It is concluded that current exchange-correlation functionals do not accurately describe the polarization of conjugated systems when the polarization is the result of donor/acceptor substitution or an external field or both.