Effective Fragment Potential method in Q-Chem: A guide for users and developers
A detailed description of the implementation of the Effective Fragment Potential (EFP) method in the Q-Chem electronic structure package is presented. The Q-Chem implementation interfaces EFP with standard quantum mechanical (QM) methods such as such as Hartree-Fock, density functional theory (DFT), perturbation theory (MP2) and coupled-cluster (CC) methods, as well as with methods for electronically excited and open-shell species, e.g., configuration interaction, time-dependent DFT, and equation-of-motion CC (EOM-CC). In addition to the QM/EFP functionality, a 'fragment-only' feature is also available (when the system is described by effective fragments only). To aid further developments of the EFP methodology, a detailed description of the C++ classes and EFP module's workflow is presented. The EFP input structure and EFP job options are described. To assist setting up and performing EFP calculations, a collection of Perl service scripts is provided. The precomputed EFP parameters for standard fragments such as common solvents are stored in Q-Chem's auxiliary library; they can be easily invoked, similarly to specifying standard basis sets. The instructions for generating user-defined EFP parameters are given. Fragments positions can be specified by Euler angle representation. The interface with the IQmol and WebMO software is also described.