Geometry optimization for large systems by the elongation method

Geometry optimization using the elongation method is developed at the Hartree–Fock level of theory. The formalism of elongation energy gradient and its accuracy have been validated by model systems calcula- tions. The linear poly-hydrogen fluoride, polyethylene, planar polyacetylene and extended polyalanine are opti- mized using different basis sets and compared with con- ventional results. The results show that the elongation Hartree–Fock geometry optimization (ELG-HF-OPT) can reproduce conventional calculation results with high accuracy for various basis sets. For the poly-hydrogen fluoride calculation at 6-31G(d,p) basis set, moreover, ELG-HF-OPT gives a structure with lower ground state energy than conventional results with the same optimiza- tion convergence threshold. This means the potential pos- sibility of ELG-HF-OPT can locate a more stable structure than conventional calculations with the same optimization convergence criteria. Therefore, the ELG-HF-OPT would provide one more choice for performing optimization on complicated large systems