Elongation method for linear scaling SCF calculations of polymers

The elongation method utilizes the locality or nearsightedness approximation in the electronic structure calculations. In analogy to polymerization/copolymerization mechanism, the elongation method extends the polymer chain by stepwise adding a monomer unit to a starting oligomer. In the same time it preserves as much chemical information as possible by freezing the shape of selected localized molecular orbitals (LMOs). As a consequence the dimension of variational space is effectively reduced and is almost constant along the whole elongation process. Besides diminishing the variational space, LMO basis can also reduce the physical dimension of the Fock matrix in a so called cutoff procedure. In such a way, the computation of large number of two-electron integrals is avoided in the SCF process. In this paper we describe the elongation method implemented with the cutoff technique at the Hartree-Fock level of theory. As an illustrative example, we have chosen C5 conformer of polyglycine to demonstrate that the elongation method is a powerful tool in electronic structure calculations.