Electronic band structure, densities of states, and the details of chemical bonding in Cu2ZnSnSe4, a compound used in photovoltaic applications and structurally close to chalcopyrite, has been studied using the SIESTA method within the local-density approximation of the density-functional theory. Calculated zone-center phonons for Cu2ZnSnSe4 in kesterite and stannite phases reveal a similarity to those in structurally close CuInSe2, with some additional modes which must become observable due to the reduced crystal symmetry. The prediction that the highest TO vibration mode has predominantly Zn contribution is consistent with a strong Zn concentration dependence of this particular mode earlier observed in
Zn,Cd-mixed kesterite systems. A detailed comparison of calculated vibration spectra for kesterite and stannite phases helps to identify the features which could be useful for distinguishing these two structures in practice.