Variational density matrix functional theory calculations with the lowest-order Yasuda functional

Fully variational density matrix functional theory calculations reveal a critical flaw in the Yasuda functional derived from the contracted Schrödinger equation and the lowest-order cumulant expansions of the reduced density matrices. Although it yields finite energies in conjunction with finite basis sets, it appears to be unbound from below even for one of the simplest two-electron systems, namely, the helium atom at the s limit, once a complete basis set is employed. This observation casts serious doubts upon its practical usefulness in electronic structure calculations.