Ab initio quantum mechanical computational techniques have been applied to questions in various molecular systems from triatomics to nucleic acid (NA) bases. For triatomic copper hydroxide (CuOH), a range of ab initio methods have been employed to investigate the ground and two lowest-lying singlet excited electronic states. The 11optimized geometrical parameters for the A' and 1 A" states agree fairly well with 1available experimental values. However, the 2 A' structure is in poor agreement with experiment. The predicted adiabatic excitation energies are also inconsistent with 11experiment for the 2 A' and 1 A" states. All theoretical methods show lower adiabatic 1-1excitation energies for the 1 A" state (53.1 kcal mol) than those for the corresponding 1-112 A' state (57.6 kcal mol), suggesting that the 1 A" state might be the first singlet 1excited state while the 2 A' state might be the second singlet excited state. Extensive ab initio methods have been used to study the triatomic PCN / PNC species and the transition state of the exothermic PNC PCN reaction. Both PCN and 3-3-PNC are linear with ground states, and linear PNC ( ) is predicted to lie 13.5 -1kcal mol [with zero-point vibrational energy (ZPVE) correction] above linear PCN ~X~X 3-3( ). The isomerization transition state is found to be cyclic PCN ( A3) with angles e (PCN) = 82.2, e (CNP) = 63.1, and e (NPC) = 34.7. The isomerization -13-barrier is predicted to be 35.7 kcal mol relative to linear PCN ( ). With respect to the NA bases, the ab initio Hartree-Fock (HF) and second-order Mller-Plesset perturbation (MP2) methods have been applied to analyze the nonplanarity of the NA base amino group. New benchmark predictions have been obtained at the cc-pCVQZ and aug-cc-pVQZ MP2 levels of theory for adenine, guanine, cytosine, thymine, and uracil. Three out of the five NA bases, namely adenine, guanine, and cytosine, are intrinsically nonplanar due to the existence of pyramidal amino groups. Guanine is much more nonplanar than adenine and cytosine. The predicted classical barriers to planarization are 0.020 (adenine), 0.742 (guanine), and 0.032 (cytosine) kcal -1mol.