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Exploring the vibrational properties of anharmonic crystals

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Calculating the electronic properties of crystals is now routine, and many software packages are available for performing these calculations. In contrast, calculations of the vibrational properties of crystals are far less developed. When vibrational properties are not neglected entirely, they are typically calculated under the harmonic approximation. The harmonic approximation makes calculation relatively straightforward and inexpensive, but for many systems this approximation is overly restrictive, and there are many crystal properties which it entirely fails to capture.

There are a number of anharmonic vibrational methods which are widely used for calculating the properties of molecular systems, and in recent years some of these methods have been adapted for use with crystals. However, crystals are not molecules. Unlike molecules, crystals have translational symmetry, and many crystals have complex space group symmetries. Further, crystals are extended systems, and so their properties must be correctly extensive or intensive. Symmetry and size-consistency can be exploited to accelerate crystal models, and any model which violates these conditions is likely to be inaccurate, yet these conditions are not widely recognised in the vibrational modelling community.

This talk seeks to highlight the importance of symmetry and size-consistency to modelling the vibrational properties of crystals, and to present a number of problems with existing models which arise where these constraints are broken.

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This talk is part of the Electronic Structure Discussion Group series.

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