Direct calculation of the one-electron density matrix for closed-shell systems

It has been found that independent parameters in the variation of a one-ele ctron density matrix (DM) for closed-shell systems are elements of its unit ary transformed matrix and, in a special case, reduce to the rotation param eters that connect the occupied and virtual orbital spaces in the exponenti al transformed self-consistent field method. To obtain the unitary matrix o f transformation, a simpler method of orthogonalizing the column vectors of the DM has been proposed instead of its diagonalization. An iterative meth od has been formulated to determine these independent parameters. Several t est calculations using this method reproduced the results using the Hartree -Fock-Roothaan method. (C) 2000 American Institute of Physics. [S0021- 9606 (00)31437-4].