Elongation method at restricted open-shell Hartree-Fock level of theory

Elongation and cutoff elongation methods are described at the restricted open-shell Hartree-Fock (ROHF) level of theory. Test calculations are performed for polyethylene radicals CH₃-(CH₂)_N-1 (N = 8, 10, 12, ⋯ 40), using minimal (STO-3G) and valence double-ζ (6-31G) basis sets. It is demonstrated that the process of diminishing the variational space in the elongation method does not introduce a large error, and that the error can be controlled by the size of the starting radical. Implementation of the cutoff technique to the elongation method leads to distinct time savings in the self-consistent field (SCF) process and does not worsen the results. In contrast to full variational space ROHF treatment, the step SCF CPU time in the cutoff elongation calculations appears to saturate to an asymptotic value that permits investigation of large molecular systems.