Potential energy surfaces of the ¹ A ₁ and ³ B ₁ states for XH₂ molecules (X=C, Si, Ge, Sn, Pb) are investigated with ab initio full valence multiconfigurational self‐consistent field wave functions, using effective core potentials. Spin–orbit coupling is also calculated to construct relativistic potential energy surfaces. The relativistic potential energy surfaces are compared with the adiabatic nonrelativistic potentials. Simple one dimensional Landau–Zener transition probabilities are calculated at the minimum energy crossing points of XH₂ molecules to estimate the intersystem crossing probability.