General second order MCSCF theory: A density matrix directed algorithm

A quadratically convergent general MCSCF algorithm is presented which is suitable for both ground state and excited state calculations. This method converges more rapidly than annihilation of singles techniques and is computationally very attractive as it does not involve the contraction of a potentially large Hamiltonian matrix on each interation. Sample calculations are performed on the two lowest states of the same symmetry for Li2, Li4, and BeO. For BeO, a symmetry restricted full valence MCSCF (81 configuration state functions) and a first order CI calculation using the MCSCF orbitals yielded good agreement with the experimental splitting of the two lowest 1Σ+ states. The results demonstrate that the algorithm described is capable of providing an efficient solution of large, general, MCSCF problems.