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The paper reviews several modern methods for calculation of Helmholtz free energy, associated with atomic vibrations: quasiharmonic approximation, self-consistent lattice dynamics method, calculation based on velocity autocorrelation function, temperature-dependent effective potential and thermodynamic integration. The results of free energy calculation by these methods are compared with each other in a wide temperature range, and their applicability is discussed. As a tool we use classical molecular dynamics modeling on the example of bcc molybdenum and fcc aluminum. The relation is observed between how the vibrational spectrum changes with temperature due to the anharmonicity of the potential and how free energy is reproduced. All methods are consistent with each other within 10 meV/at. at relatively low temperatures. At high temperatures, the discrepancy reaches tens of meV/at., although the relative error is less than 5%. [ABSTRACT FROM AUTHOR]

This article discusses a general method for constructing interatomic potentials based on truncated Taylor series expansion. Specifically, it addresses the scope of application of the method, and demonstrates its practical importance in capturing anharmonicity for a Lennard–Jones solid. In particular, the third-order terms in the truncated potential are shown to accurately approximate the thermal conductivity of the standard interaction Lennard–Jones potential. The paper also describes an efficient algorithm for locating the equilibrium lattice site of an atom in a three-dimensional crystal lattice displaced from its equilibrium position. [ABSTRACT FROM AUTHOR]

Especially for larger molecules relevant to life sciences, vibrational self-consistent field (VSCF) calculations can become unmanageably demanding even when only first and second order potential coupling terms are considered. This paper investigates to what extent the grid density of the VSCF's underlying potential energy surface can be reduced without sacrificing accuracy of the resulting wavenumbers. Including single-mode and pair contributions, a reduction to eight points per mode did not introduce a significant deviation but improved the computational efficiency by a factor of four. A mean unsigned deviation of 1.3% from the experiment could be maintained for the fifteen molecules under investigation and the approach was found to be applicable to rigid, semi-rigid and soft vibrational problems likewise. Deprotonated phosphoserine, stabilized by two intramolecular hydrogen bonds, was investigated as an exemplary application. [ABSTRACT FROM AUTHOR]

[Display omitted] The 2ν 3 -ν 4 difference bands of 14NO 3 and 15NO 3 have been observed in the 1643.6 cm−1 and 1622.0 cm−1 region, respectively, and analyzed to determine molecular constants in the v 3 = 2 state. The present paper also reports re-analysis of high-resolution spectra of the 15NO 3 radical from 2000 to 2500 cm−1, which have been reported previously without vibrational assignments [ J. Phys. Chem. A 114(2010), 980]. Three bands at 2004, 2128, and 2492 cm−1 are analyzed as the ν 1 + ν 3 , 2ν 3 , and ν 1 + ν 3 + ν 4 bands, including perturbations from nearby states. All vibrational states of 14NO 3 and 15NO 3 so far observed by infrared spectroscopy and laser induced fluorescence (LIF) are analyzed by using a vibrational energy matrix including cubic and quartic anhamonic effects, to determine anharmonicity constants and eigenvector coefficients. The first-order Coriolis coupling constants C ζ and the effective spin–orbit interaction constants a eff are calculated by using the eigenvector coefficients and compared with the observed values. [ABSTRACT FROM AUTHOR]

Nitroxide radicals are characterized by a long-lived open-shell electronic ground state and are strongly sensitive to the chemical environment, thus representing ideal spin probes and spin labels for paramagnetic biomolecules and materials. However, the interpretation of spectroscopic parameters in structural and dynamic terms requires the aid of accurate quantum chemical computations. In this paper we validate a computational model rooted into double-hybrid functionals and second order vibrational perturbation theory. Then, we provide reference quantum chemical results for the structures, vibrational frequencies and other spectroscopic features of a large panel of nitroxides of current biological and/or technological interest. [ABSTRACT FROM AUTHOR]

Recent schemes for the efficient calculation of vibrational mode frequencies and lifetimes in weakly anharmonic systems such as solid crystal structures have shown promise, with simple relationships being observed between low order moments of the Liouvillian and the observed lifetime. While only shown empirically, two parameters seemed sufficient to determine the lifetime to high accuracy over a large range of temperatures for a few simple low degree of freedom models and a Lennard–Jones solid. In this paper we analytically reproduce the numerically observed relationship using a few generally applicable assumptions about the density of states. This advance is made by studying the autocorrelation based on an "ensemble eigenstate" of the Liouvillian. We propose the use of this ensemble eigenstate as the basis for autocorrelations to simplify calculations in richer systems. An analysis is also performed of the relationship between canonical and microcanonical autocorrelation functions and a long standing error in the long time behavior of such functions originally presented by Sen, Sinkovits, and Chakravarti is corrected. [ABSTRACT FROM AUTHOR]