Fractional occupation number approaches for CAS (2,2) systems based on second‐order density

Abstract

AbstractIn previous work, we applied a fractional occupation number (FON) approach within density functional theory (DFT) for twisted methylene and found that the applicability of FON‐DFT is limited for the neighborhood of the completely degenerate state, implying the necessity of the second‐order density (SOD) expression of the energy functional. In the succeeding work, we have derived the exact expression of SOD of CAS (2,2) by the natural orbitals (NOs) and their occupation numbers, which we call “one‐body representation of second‐order density” (ORSOD). We present here the FON scheme based on ORSOD by directly optimizing the NOs and their occupation numbers. In contrast to the previous FON‐DFT, this FON scheme is found to work well, in particular, the dissociation limit of the heteronuclear systems, showing that this scheme can be an alternative for complete active space self‐consistent field (2,2) methods for such cases. The current scheme provides a guideline for extension and approximations of the FON scheme to investigate mesoscopic systems with large active space. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005