Do Water Molecules Always Stabilize Resonances? Microhydration Effects on Thymine Shape Resonances

We investigate microhydration effects on the three low-lying π* shape resonances of thymine using the Resonance via Padé approach in combination with the EA-EOM-DLPNO-CCSD method. For isolated thymine, the calculated resonance positions are benchmarked against projected CAP-EA-EOM-CCSD calculations and compared with available theoretical and experimental data. Upon hydration, the 1π* and 2π* resonances undergo systematic stabilization accompanied by a significant increase in their lifetimes, whereas the 3π* resonance exhibits a more complex behavior. In particular, the lifetime of the lowest resonance increases from 39 fs in isolated thymine to 110 fs in the thymine(H2O)3 cluster. A detailed analysis reveals that the observed resonance shifts arise from competing contributions involving hydrogen bonding, electrostatic interactions, microsolvation induced geometric distortion, and finite basis set effects. Ghost atom calculations demonstrate that diffuse basis functions associated with nearby water molecules contribute appreciably to the apparent stabilization, while explicit inclusion of water molecules leads to genuine physical stabilization of the resonance states. Furthermore, calculations on multiple conformers of the monohydrated cluster show that resonance positions and lifetimes depend strongly on the local hydrogen-bonding arrangement and microsolvation geometry. These findings demonstrate that resonance stabilization in microhydrated nucleobases is governed by a subtle interplay between geometry, basis set effects, and intermolecular interactions.