Subsystems of many-electron system and reduced density matrices

摘要

Dividing a system into subsystems is a widely used approach that allows one to calculate the electronic structure of large and complex systems. Quite often, the first order reduced density matrix of the system is employed in this approach. Unfortunately, it turns out that the obtained values of the electronic populations of subsystems, which must correspond to the number of electrons in the subsystem, are fractional and they noticeably deviate from integers. In the present paper for a system in the state of a particular type it is shown that if the second order reduced density matrix is also taken into account in the subsystem generations, then the orthogonal one electron basis can be found with which the calculated populations of subsystems will be practically equal to integer numbers. The said state of a particular type is the state whose wave function is a single determinant with doubly occupied orbitals. This is a reasonable approximation to the wave function for the singlet ground state of a standard atomic-molecular system.