Geometry optimizations were also performed for verification. In Table , optimized bond distances with the IPEA shift are summarized. As the error for S 0 gradients (Table ) was rather small, the optimized bond distances (and energies) for all gradients agreed. On the other hand, geometry optimizations using approximate gradients for S 1 did not fully converge. The number of independent vectors was constant during geometry optimizations, indicating that the failure of the geometry optimization was caused by the inaccuracy of the gradient. The distances obtained with fully analytic gradients reproduced those obtained with numerical gradients.