# Formula Thermal Capacity (Einstein Approximation) Einstein temperature    Temperature    Number of particles ## Thermal capacity

Thermal capacity indicates how well a material can store thermal energy at a constant volume $$V$$. It is the derivative of the internal energy with respect to temperature.

The thermal capacity in the Einstein approximation is well suited when optical phonons dominate, because in contrast to acoustic phonons, they have a relatively flat dispersion relation. The unit of thermal capacity is $$\mathrm{J}/\mathrm{K}$$.

## Einstein temperature

Unit
Einstein temperature of the considered crystal for which this temperature is characteristic. It is defined as $$T_{\text E} = \frac{\hbar \, \omega_{\text E}}{k_{\text B}}$$, where $$\omega_{\text E}$$ is the Einstein frequency. It is constant and is chosen to fit the material. In the Einstein approximation it is assumed that all $$3N$$ vibrational states of the crystal have the same frequency, namely the Einstein frequency $$\omega_{\text E}$$.

## Temperature

Unit
Absolute temperature of the crystal under consideration.

## Number of particles

Unit
Number of particles in the crystal.

## Boltzmann Constant

Unit
Boltzmann constant is a natural constant, which appears frequently in statistical physics and thermodynamics. It has the value: $$k_{\text B} ~\approx~ 1.380 \,\cdot\, 10^{-23} \, \frac{\text J}{\text K}$$.