Crossover between collective and independent-particle excitations in quasi-2D electron gas with one filled subband

While it has been recently demonstrated that, for quasi-two-dimensional electron gas (Q2DEG) with one filled subband, the dynamic exchange f_x and Hartree f_H kernels cancel each other in the low-density regime r_s →∞ (by half and completely, for the spin-neutral and fully spin-polarized cases, respectively), here we analytically show that the same happens at arbitrary densities at short distances. This motivates us to study the confinement dependence of the excitations in Q2DEG. Our calculations unambiguously confirm that, at strong confinements, the time-dependent exact exchange excitation energies approach the single-particle Kohn–Sham ones for the spin-polarized case, while the same, but less pronounced, tendency is observed for spin-neutral Q2DEG.