We evaluate the density matrix of an arbitrary quantum mechanical system in terms of the quantities pertinent to the solution of the time-dependent density functional theory (TDDFT) problem. Our theory utilizes the adiabatic connection perturbation method of G{o}rling and Levy, from which the expansion of the many-body density matrix in powers of the coupling constant $lambda$ naturally arises. We then find the reduced density matrix $rho_lambda({bf r},{bf r},t)$, which, by construction, has the $lambda$-independent diagonal elements $rho_lambda({bf r},{bf r},t)=n({bf r},t)$, $n({bf r},t)$ being the particle density. The off-diagonal elements of $rho_lambda({bf r},{bf r},t)$ contribute importantly to the processes, which cannot be treated via the density, directly or by the use of the known TDDFT functionals. Of those, we consider the momentum-resolved photoemission, doing this to the first order in $lambda$, i.e., on the level of the exact exchange theory. In illustrative calculations of photoemission from the quasi-2D electron gas and isolated atoms, we find quantitatively strong and conceptually far-reaching differences with the independent-particle Fermis golden rule formula.
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