Efficient EOM-CC-based Protocol for the Calculation of Electron Affinity of Solvated Nucleobases: Uracil as a Case Study

Abstract

We present an explicit solvation protocol for the calculation of electron affinity values of the solvated nucleobases. The protocol uses a quantum mechanics/molecular mechanics (QM/MM) approach based on the newly implemented domainbased pair natural orbital EOM-CCSD (equation-of-motion coupled-cluster single−double) method. The stability of the solvated nucleobase anion is sensitive to the local distribution of the water molecules around the nucleobase, and the calculated electron affinity values converge slowly with respect to the number of snapshots and the size of the water box. The use of nonpolarizable water molecules leads to an overestimation of the electron affinity and makes the result sensitive to the size of the QM region in the QM/MM calculation. The electron affinity values, although sensitive to the size of the basis set, lead to an almost constant blue shift of the electron affinity upon the increase in the basis set. The present protocol allows for a controllable description of the various parameters affecting the electron affinity value, and the calculated adiabatic electron affinity values are in excellent agreement with experimental results.

Electron Attachment to Cytosine: The Role of Water

“…These 55 snapshots were separately optimized for the neutral as well as anion geometry using QM/MM method. The details of the protocol for the calculation of the adiabatic electron affinity of the solvated nucleobases can be found in reference 46 .…”

“…However, the effect of the long-range part of the polarization 63 is generally small. The calculation of final average VDE of cytosine solvated in liquid water as defined in the protocol described ref46 :Final average VDE= Average VDE of 400 snapshots from the EA-EOM-DLPNO-CCSD/augcc-pVDZ based QM/MM calculation + basis set correction+ correction for the finite number of water molecules + correction for the size of the QM region=8.24 eV +0.29 eV +0.66 eV-1.58 eV= 7.61 eV…”

Probing Interfacial Effects on Ionization Energies: The Surprising Banality of Anion-Water Hydrogen Bonding at the Air/Water Interface

“…82,83,87 Long-range polarization does change dramatically upon ionization, however, leading to very slow convergence of VIEs with respect to the number of explicit water molecules. [87][88][89][90][91][92][93][94] Consideration of the Born ion model suggests ∼ R −1 convergence behavior, for an ion-water cluster of radius R, and in practice that may mean 500-1000 water molecules to obtain converged results. [88][89][90][91][92][93][94] This problem is not unique to VIEs and is also encountered in pK a calculations.…”

“…[87][88][89][90][91][92][93][94] Consideration of the Born ion model suggests ∼ R −1 convergence behavior, for an ion-water cluster of radius R, and in practice that may mean 500-1000 water molecules to obtain converged results. [88][89][90][91][92][93][94] This problem is not unique to VIEs and is also encountered in pK a calculations. 104 A solution to this conundrum is to use continuum boundary conditions to accelerate convergence with respect to the size of the atomistic solvent cluster.…”

“…That issue is examined here by computing bulk and interfacial VIEs for 16 common inorganic ions for which experimental data (from liquid microjets) are available, 59,86 using a combination of MD simulations and electronic structure calculations. Long-range polarization makes a significant contribution to VIEs in water, [87][88][89][90][91][92][93][94][95] and we incorporate this effect via dielectric continuum boundary conditions around an electronic structure calculation that includes several solvation shells of explicit water molecules. 95 Excellent agreement with experimental results suggests that our procedure constitutes a useful, general approach for modeling aqueous VIEs.…”