A few remarks concerning application of the Lifshitz theory to calculation of the Casimir–Polder interaction

The Lifshitz theory provides a semiclassical description of the Casimir–Polder atom-plate interaction, where the electromagnetic field is quantized, whereas the material of the plate is considered as a continuous medium. This places certain restrictions on its application regarding the allowable atom-plate separation distances and the dielectric properties of the plate material. Below, we demonstrate that in some recent literature the application conditions of the Lifshitz theory established by its founders are violated by applying it at too short separations and using the dielectric permittivities possessing the negative imaginary parts in violation of the second law of thermodynamics.