Unified Framework for Molecular Response Functions of Different Electronic-Structure Models

A unified framework─SymResponse─has been developed as a versatile tool to aid the implementation of response theory for different electronic-structure models. The framework manipulates the quasi-energy formulation of response theory at a symbolic level by building on top of other well-developed symbolic libraries. Response functions can therefore be nicely represented by "symbolic expressions," which can be further evaluated numerically by users by developing their own evaluation routines with the assistance of SymResponse. The design of SymResponse makes it extensible to different electronic-structure models with only a moderate further amount of development effort. Response theory at Hartree-Fock, density functional theory, and coupled-cluster levels has been implemented in the present work as a demonstration, where elimination rules [Kristensen, K. <i>J. Chem. Phys.</i> 2008, 129, 214103] can also be applied to offer the possibility to reduce the size of computational tasks.