PhononA, Harmonicity & Anharmonicity Software, ver. 7.10
Phonon, Harmonic Software, ver. 6.15

Temperature of solids is almost entirely determined by phonons. Phonons, in turn, are defined as a combination of atomic vibrations. Small atomic vibrational amplitudes lead to low temperature of the material. High amplitudes involve high temperature. Since the ambient temperature is always finite, all atoms of the whole world vibrate all the time. Thus, phonons are everywhere in solids.

Therefore, phonons are responsible for thermal expansion, contribute to appearance of phase transitions, and phase diagrams, phonons play a role in elastic and dielectric constants, transport processes, such as diffusion and chemical reactions, catalysis, superconductivity, ferroelectricity, shape memory alloys, etc. Moreover, phonon properties can be substantially modified when the crystal is decorated by defects, like additional atoms, precipitates, interfaces, grain boundaries, or surfaces, etc.

Phonons are specified in the lattice dynamics theory. This theory, due to fast and powerful computers, allows to compute phonon characteristics using first-principle, or ab initio methods. Hence, at present phonons in many systems, can be computed without any adjustable parameters, and in this way the calculated lattice dynamics becomes a complementary tool to the measurements.

Anharmonicity exists in any material. It is responsible for establishing the thermal equilibrium in the system. The small anharmonicity can be handled within the perturbation theory. The strong anharmonicity would, however, require a summation over many anharmonic terms, which provide obvious difficulties. The anharmonic influences are seen in macroscopic quantites, specially in thermodynamical quantities, moreover, they may severly modify the neutron, x-ray scattering properties, infrared and Raman spectra. A specially important is the appearance of electron-phonon coupling effect related also to anharmonicity. The PhononA Software does not use the perturbatiion approach, but it applies an original way of probing the atomic patterns arrising during anharmonic displacements. In this manner it includes, at least partly, the many body anharmonic interaction up to a considerable order.

The Phonon and PhononA softwares allow to calculate the phonon characteristics. As an input this softwares require to know (i) a crystal structure, and (ii) a file of Hellmann-Feynman forces. External ab initio program like VASP, Wien2k, or MedeA of Materials Design can be used in this respect.