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 calculations. The linear poly-hydrogen fluoride, polyethylene, planar polyacetylene and extended polyalanine are optimized using different basis sets and compared with conventional 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 optimization convergence threshold. This means the potential possibility 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.