Circulant orbitals and the electron-correlation problem.

Circulant, as well as canonical, orbitals are used in the different orbitals for different spins method for treating electron correlation. Circulant orbitals provide a theoretical justification for the use of a single parameter, even when the canonical orbitals have widely different orbital energies. Illustrative calculations on the ground state of the Be atom show the importance of choosing the correct "pairs" in the method. A two-parameter version of the conventional method gives 74% of the improvement obtained by a full configuration-interaction treatment using 20 linear parameters, while a one-parameter linear combination of two different coupling schemes of the circulant method gives approximately 61%. The latter wavefunction provides a compact description of the electron correlation.