Effective fragment potential method in Q-CHEM: a guide for users and developers.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Electronic Structure Theory

Background:

• The effective fragment potential (EFP) method offers a computationally efficient approach for modeling large molecular systems.
• Integrating EFP with standard quantum mechanical (QM) methods is crucial for accurate and scalable simulations.

Purpose of the Study:

• To present a comprehensive description of the EFP method's implementation within the Q-CHEM electronic structure package.
• To detail the interfacing of EFP with various QM methods and its application to diverse chemical species.
• To provide guidance for users on setting up and performing EFP calculations.

Main Methods:

• Implementation of the EFP method in Q-CHEM, interfacing with Hartree-Fock, DFT, perturbation theory, and coupled-cluster methods.
• Integration with methods for excited and open-shell states, including CI, TD-DFT, and EOM-CC.
• Development of a "fragment-only" feature for systems described solely by effective fragments.
• Detailed description of C++ classes, module workflow, input structure, and job options.

Main Results:

• Successful integration of EFP with a wide range of QM methods in Q-CHEM.
• Availability of precomputed EFP parameters for common fragments, analogous to basis sets.
• Provision of Perl scripts to facilitate EFP calculation setup and execution.
• Instructions for generating user-defined EFP parameters and specifying fragment positions.

Conclusions:

• The Q-CHEM implementation provides a robust and versatile platform for QM/EFP calculations.
• The detailed documentation and provided scripts lower the barrier for utilizing EFP in computational studies.
• This work significantly advances the applicability of EFP for complex molecular simulations.