Many scientific applications tend to perform high-volume data storage, data retrieval, and data processing, all of which demand high performance from the I/O subsystem. The focus and contribution of this work is to study the I/O behavior of the Hartree-Fock (HF) method using PAS SION. HF's I/O phases can contribute up to 62.34% of the total execution time. The authors reduce the execution time and I/O time up to 54% and 6%, respectively, of that of the original case through PASSION and its optimiza tions. Additionally, the authors categorize the factors that affect the I/O performance of HF into key application- related parameters and key system-related parameters. Based on extensive empirical results and within the ex perimental space presented in this paper, the authors orderthe parameters according to the their impact on HF's I/O performance as follows: efficient interface, prefetching, buffering, number of I/O nodes, striping factor, and striping unit. The authors conclude that application-related factors have a more significant effect on HF's I/O performance than the system-related factors within the experimental space presented in this paper.