# Combined ab initio complete-active-space and on-top-pair-density-mediated density-functional approach for efficient calculation of potential energy surfaces of excited molecular states

**Combined ab initio complete-active-space and on-top-pair-density-mediated density-functional approach for efficient calculation of potential energy surfaces of excited molecular states**

**Research Project Objectives/Research hypothesis**

The aim of the proposed research is the development of an efficient theoretical method CAS+DFT for the calculation of potential energy surfaces (PESs) of important low-lying molecular electronic states. The proposed method combines the *ab initio* complete active space (CAS) approach and density functional theory (DFT).

The research hypotheses of the proposal are aimed at the solution of the multi-coordinate and multi-effect problem of PES calculation by an effective separation of the relevant electronic interaction effects and electron coordinates with the subsequent use of reduced one-electron functionals. The following hypotheses are put forward: 1) The hypothesis of an effective separation of a relatively small active space of electron configurations associated with valence bonds (including their stretching) to calculate the major part of electron interaction. 2) The hypothesis of an effective factorization of the remainder dynamical correlation. It is hypothesized, that this type of correlation can be effectively represented as a product of a state-non-specific functional of the total dynamical correlation and a state-specific local scaling function. 3) The hypothesis of an efficient evaluation of total dynamical electron correlation with DFT. 4) The hypothesis of the efficient description of the local suppression of dynamical correlation with a scaling function, which depends on the on-top pair density.

**Research project methodology**

The underlying scientific methodology of the proposed research is the multi-scale modeling and simulation approach applied to the electron configuration space. At the core level (active system) a small active space of electron configurations associated with valence bonds is treated with the comprehensive non-empirical CAS method. At a higher level (environment) a much larger space of electron configurations associated with dynamical electron correlation is treated in a reductionist way with a DFT functional of the electron density. This results in a great, and yet efficient and physically justified, reduction of information contained in a generic quantum-mechanical object, the wave function of a given excited state.

To treat electron correlation at the one-electron level, a methodology of effective separation of the electron interaction effects is employed. It is manifested in the effective factorization of short-range dynamical correlation and the effect of its suppression with non-dynamical correlation. This factorization makes possible a physically justified restricted application of DFT to excited states.

**Expected impact of the research project on the development of science**

An efficient method of calculation of PESs of important lowest excited molecular states, the ultimate goal of the present project, would find its immediate application to advance theoretical description in photochemistry and electron spectroscopy. In a broader perspective, it can serve as an efficient theoretical tool for rational design of new functional materials with prescribed excitation characteristics. Other broader applications are theoretical description of molecular interaction with light signals in non-linear optics and nano-electronics as well as theoretical description and design of energy/charge transfer processes for sustainable energy utilization and storage.