where rs(t) is position of atom s in time t, with crystallographic position R, νs is its polarization vector associated with phonon mode ν with frequency ων and all is considered in the q-point of the space of wave vectors. With one of the part (real or imaginary) of polarization vector equal to zero, atoms vibrate along the line with frequency ω and direction given by the polarization vector. When both parts are non-zero, the atomic trajectory is an ellipsoid. If additionally both parts are equal, the ellipsoid becomes a circle. Such a case has been found in many q points of phonon dispersion of RbBi2. Two examples are shown in Fig. ). At the K point, Rb atoms are moving circularly, while Bi atoms move along a line. At the W point, all atoms are moving along a circle/ellipse. These modes might be candidates for chiral phonons which arise due to symmetry of the crystal structure and have been recently found in ABi compounds, including RbBi and CsBi (the occurrence of chiral phonons in ABi2 compoinds is discussed in more detail in ref. ).