Coupled-cluster based approach for core-level states in condensed phase: Theory and application to different...

A theoretical approach for calculating core-level states in condensed phase is presented. The approach is based on the equation-of-motion coupled-cluster (EOM-CC) theory and effective fragment potential (EFP) method. By introducing approximate treatment of double excitations in the EOM-CC with single and double substitutions ansatz, we address poor convergence issues that are encountered for the core-level states and significantly reduce computational costs. While the approximations introduce relatively large errors in the absolute values of transition energies, the errors are systematic. Consequently, chemical shifts, changes in ionization energies relative to reference systems, are reproduced reasonably well. By using different protonation forms of solvated glycine as a benchmark system, we show that our protocol is capable of reproducing the experimental chemical shifts with a quantitative accuracy. The results demonstrate that chemical shifts are very sensitive to the solvent interactions and that explicit treatment of a solvent, such as within EFP framework, is essential for achieving quantitative accuracy.