Quantum Computation with Effective Fragment Potential

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DESCRIPTION (provided by applicant): This Phase I proposal seeks support for implementing effective fragment potential (EFP) method [J. Phys. Chem. A, v.105 p.293 (2001)] in the Q-Chem electronic structure program. The EFP approach enables 1 to treat large systems with localized interactions by separating them into a small "important" part (e.g., reaction center) treated quantum mechanically (QM), and the environment, which is further subdivided into the so-called effective fragments (EFs). The interaction between the QM part and EFs is described based on multipolar expansions of EF's electronic density, and is included through the modified one-electron integrals. Conceptually, the EFP method is similar to the popular QM/MM (molecular mechanics) scheme, however, it replaces empirical MM force fields by rigorous interactions derived from QM calculations of individual fragments. Once the necessary parameters of EFs are pre-computed and stored in an auxiliary database, the cost of an EF calculation is very similar to that of a QM/MM one. The implementation of the EFP method in Q-Chem will enable the researchers to apply advanced QM methods (e.g., equation-of-motion and coupled-cluster methods) to study opens-shell and electronically excited centers in biological molecules, solutions, and materials.