Journal•ISSN: 0301-0104

# Chemical Physics

About: Chemical Physics is an academic journal. The journal publishes majorly in the area(s): Excited state & Ab initio. It has an ISSN identifier of 0301-0104. Over the lifetime, 10434 publication(s) have been published receiving 235659 citation(s).

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TL;DR: In this article, a method is presented which utilizes the calculation of the molecular electrostatic potential or the electric field at a discrete number of preselected points to evaluate the environmental effects of a solvent on the properties of a molecular system.

Abstract: A method is presented which utilizes the calculation of the molecular electrostatic potential or the electric field at a discrete number of preselected points to evaluate the environmental effects of a solvent on the properties of a molecular system. No limitations are imposed on the composition and dimension of the solute, on the goodness of the corresponding wavefunction, or on the shape of the cavity in the dielectric. Several levels of approximation, which evidence the effect of self-polarization of the system of surface charges, the influence of the tails of the solute charge distribution going beyond the limits of the cavity, and the effect of the polarization of the solute, are examined and discussed.

6,963 citations

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TL;DR: In this article, a density matrix formulation of the super-CI MCSCF method is presented, where the MC expansion is assumed to be complete in an active subset of the orbital space, and the corresponding CI problem is solved by a direct scheme using the unitary group approach.

Abstract: A density matrix formulation of the super-CI MCSCF method is presented. The MC expansion is assumed to be complete in an active subset of the orbital space, and the corresponding CI secular problem is solved by a direct scheme using the unitary group approach. With a density matrix formulation the orbital optimization step becomes independent of the size of the CI expansion. It is possible to formulate the super-CI in terms of density matrices defined only in the small active subspace; the doubly occupied orbitals (the inactive subspace) do not enter. Further, in the unitary group formalism it is straightforward and simple to obtain the necessary density matrices from the symbolic formula list. It then becomes possible to treat very long MC expansions, the largest so far comprising 726 configurations. The method is demonstrated in a calculation of the potential curves for the three lowest states (1Σ+g, 3Σ+u and 3Πg) of the N2 molecule, using a medium-sized gaussian basis set. Seven active orbitals were used yielding the following results: De: 8.76 (9.90), 2.43 (3.68) and 3.39 (4.90) eV; re: 1.108 (1.098), 1.309 (1.287) and 1.230 (1.213) A; ωe: 2333 (2359), 1385 (1461) and 1680 (1733) cm−1, for the three states (experimental values within parentheses). The results of these calculations indicate that it is important to consider not only the dissociation limit but also the united atom limit in partitioning the occupied orbital space into an active and an inactive part.

2,821 citations

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TL;DR: In this article, the temperature dependence of positron lifetimes in both the brittle and plastic phases of trimethylacetic (pivalic) acid was examined using the positron lifetime technique.

Abstract: The temperature dependence of positron lifetimes in both the brittle and plastic phases of trimethylacetic (pivalic) acid 1 has been examined using the positron lifetime technique. In the plastic phase two long-lived components attributable to ortho-positronium (ortho-Ps) decay are observed. The longer of these (≈2.8 ns) increases with temperature and is believed to be characteristic of ortho-Ps trapped at defects, probably mono- and di-vacancies. A shorter lifetime component (≈1 ns) which shows little temperature dependence is also present in the brittle phase. Additionally in the brittle phase and close to the transition region a longer lifetime is detected. This may also be associated with trapping of ortho-Ps in thermally created defects, probably vacancies. In the plastic phase the intensity of a short-lived component associated with para-Ps is confirmed to be approximately one third of the total intensity of the two long-lived components associated with ortho-Ps, but in the brittle phase it higher than one third. On using data for several plastic crystals a relationship is established between the lifetime of ortho-Ps trapped in a vacancy and the vacancy volume.

1,528 citations

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TL;DR: In this article, the chain-of-spheres exchange (COSX) algorithm was proposed to speed up Hartree-Fock and hybrid density functional calculations by forming the Coulomb and exchange parts of the Fock matrix by different approximations.

Abstract: In this paper, the possibility is explored to speed up Hartree–Fock and hybrid density functional calculations by forming the Coulomb and exchange parts of the Fock matrix by different approximations For the Coulomb part the previously introduced Split-RI-J variant (F Neese, J Comput Chem 24 (2003) 1740) of the well-known ‘density fitting’ approximation is used The exchange part is formed by semi-numerical integration techniques that are closely related to Friesner’s pioneering pseudo-spectral approach Our potentially linear scaling realization of this algorithm is called the ‘chain-of-spheres exchange’ (COSX) A combination of semi-numerical integration and density fitting is also proposed Both Split-RI-J and COSX scale very well with the highest angular momentum in the basis sets It is shown that for extended basis sets speed-ups of up to two orders of magnitude compared to traditional implementations can be obtained in this way Total energies are reproduced with an average error of

1,402 citations

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TL;DR: In this article, the point dipole interaction model for molecular polarizability is modified by replacing the point-dipole interaction by an interaction between smeared out dipoles, and rules are developed to indicate plausible forms for this modified interaction.

Abstract: The point dipole interaction model for molecular polarizability recently proposed by Applequist, Carl, and Fung is modified by replacing the point dipole interaction by an interaction between smeared out dipoles. Rules are developed to indicate plausible forms for this modified interaction. The polarizabilities of a wide range of chemically different molecules can be calculated, using for each atom one polarizability independent of its chemical enviromnent. The errors are comparable to experimental uncertainty. Special care is taken to produce a model that tends to avoid infinite polarizabilities without use of cutoffs at short distances.

1,062 citations