Two potential energy curves for the ground electronic state X²Π and the first excited electronic state A²Σ⁺ of OH radical have been calculated using the internally contracted multiconfiguration-reference configuration interaction (IC-MRCI) method including Davidson correction. And they were fitted to analytical potential energy functions using the Murrell-Sorbie potential function to deduce the spectroscopic parameters: equilibrium bond length R_e, rotation coupling constant ω_e, anharmonic constant ω_eχ_e, equilibrium rotation constant B_e and vibration-rotation coupling constant α_e. These constants and higher-order anharmonic constant ω_eY_e were also calculated by POLFIT. Most of the values obtained are in excellent agreement with experimental results. The calculated dissociation energy D₀ for OH(X²Π) is 35568.86 cm^－1, which is in excellent agreement with the recent experimental value (35565±30) cm^－1. The calculated dissociation energy D₀ for OH(A²Σ⁺) is 18953.93 cm^－1. The calculated vertically excited energy from the ground state X₂Π (v＝0) to the first excited state A²Σ⁺ (ν＝0) is 32496.42 cm^－1.

Key words: OH radical, IC-MRCI, spectroscopic parameter, dissociation energy