A precaution for the hybrid density functional calculation of open-shell systems

We show that a naive treatment of open-shell systems in hybrid density functional calculations ignoring the spin dependence causes significant errors due to a kind of self interaction that is not emerged in spin-dependent calculations. As numerical examples, we compare the results of the LDA, GGA, and PBE0 calculations on the ionization potential and electron affinity of C₆₀ molecule and the GGA and HSE calculations on the singly charged monovacancy in crystalline Si.