Some low-lying states of the nine-valence-electron systems HBP and HPB radicals have been studied using three methods CASSCF, CASPT2, and B3LYP with the contracted atomic natural orbital (ANO) and cc-pVTZ basis sets. The geometries of all stationary points along the potential energy surfaces were optimized at the CASSCF/ANO, CASPT2/ANO, and B3LYP/cc-pVTZ levels. The potential energy curves of isomerization reactions between HBP and HPB were calculated as a function of HBP bond angle. The calculated results indicated that the ground-state HBP is linear, while the ground-state HPB is bent, which is in contradiction to Walsh's rules predicting linear structures for the HXY systems containing 10 or less valency electrons. (c) 2012 Elsevier B.V. All rights reserved.