Q-Chem/CP2K software interface for projection-based, pseudoperiodic wavefunction-in-DFT embedding

We report an implementation of projection-based quantum embedding that combines periodic density functional theory in the CP2K code with correlated wavefunction calculations in the Q-Chem program. Using this interface, correlated wavefunction methods can be applied to a subset of molecular orbitals, selected in an automated manner based on a user-defined list of nuclei, with an embedding potential that provides electronic coupling to the remaining orbitals that comprise the environment. Our implementation uses exact projection, without the need for any level-shift operator, and is "pseudoperiodic" in the sense that periodic boundary effects are implicit in the orbitals used in the (non-periodic) wavefunction calculation. Convergence tests demonstrate that computed properties are faithful to the corresponding periodic quantities, provided that the high-level subsystem is spatially localized and small, relative to the periodic simulation cell. Spectroscopic examples for aqueous chromophores demonstrate converged results using a very limited subset of water molecules in the simulation cell, decoupling the choice of excited-state method and basis set from the functional that is used to propagate ground-state ab initio molecular dynamics. Solvatochromic shifts for aqueous uracil are converged using equation-of-motion coupled-cluster theory, without the need for solvent molecules in the wavefunction calculation, and the embedded calculation captures solvent effects that are absent in gas-phase "microhydration" studies. Additional applications include adsorption of CO on a slab model of Si(100) and bond dissociation in a one-dimensional periodic polymer. In both of the latter cases, the system/environment partition crosses a covalent bond; these applications demonstrate that pseudoperiodic embedding can be applied to realistic systems in materials science.